docs: guide to installables docs in installable commands' docs

The installables syntax is not documented in any of the man pages or
docbook pages for any of those individual commands. And while these
commands really should at least peripherally individually document how
installables work, in the meantime we can at least direct people to the
right place.

This commit also clarifies the unexpected fact that `nix profile remove`
and `nix profile upgrade` do *not* take installables.

Change-Id: I3b1453cb197a613bbab639c66a466365c3592c6d

src/nix/build.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Build the default package from the flake in the current directory:

src/nix/bundle.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Bundle Hello:

src/nix/derivation-add.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Description
 
 This command reads from standard input a JSON representation of a

src/nix/derivation-show.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Show the [store derivation] that results from evaluating the Hello

src/nix/develop.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Start a shell with the build environment of the default package of

src/nix/diff-closures.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Show what got added and removed between two versions of the NixOS

src/nix/edit.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Open the Nix expression of the GNU Hello package:

src/nix/eval.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Evaluate a Nix expression given on the command line:

src/nix/fmt.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 With [nixpkgs-fmt](https://github.com/nix-community/nixpkgs-fmt):

src/nix/log.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Get the build log of GNU Hello:

src/nix/nix.md

@@ -162,7 +162,7 @@ The Nix expression in that file, or any selected attribute, must evaluate to a d
 
 Example: `--expr 'import <nixpkgs> {}' hello`
 
-When the option `--expr` *expression* \[*attrpath*...\] is given, installables are interpreted as the value of the of the Nix expression.
+When the option `-E` / `--expr` *expression* \[*attrpath*...\] is given, installables are interpreted as the value of the of the Nix expression.
 If attribute paths are provided, commands will operate on the corresponding values accessible at these paths.
 The Nix expression, or any selected attribute, must evaluate to a derivation.
 

src/nix/path-info.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Print the store path produced by `nixpkgs#hello`:

src/nix/print-dev-env.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Apply the build environment of GNU hello to the current shell:

src/nix/profile-install.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Install a package from Nixpkgs:

src/nix/profile-remove.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note**: unlike [`nix profile install`](./nix3-profile-install.md), this command does *not* take installables.
+
 # Examples
 
 * Remove a package by position:

src/nix/profile-upgrade.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note**: unlike [`nix profile install`](./nix3-profile-install.md), this command does *not* take installables.
+
 # Examples
 
 * Upgrade all packages that were installed using an unlocked flake

src/nix/repl.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Display all special commands within the REPL:

src/nix/run.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Run the default app from the `blender-bin` flake:

src/nix/search.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Show all packages in the `nixpkgs` flake:

src/nix/shell.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Start a shell providing `youtube-dl` from the `nixpkgs` flake:

src/nix/why-depends.md

@@ -1,5 +1,7 @@
 R""(
 
+**Note:** this command's interface is based heavily around [*installables*](./nix.md#installables), which you may want to read about first (`nix --help`).
+
 # Examples
 
 * Show one path through the dependency graph leading from Hello to