docs: document lookupFileArg syntax command refs

Change-Id: Ib6d68594a16132805ba5d97526e16f7b3633117e

doc/manual/src/command-ref/fileish-summary.md

@@ -0,0 +1,8 @@
+<!-- File-ish argument syntax summary. This file gets included into pages like nix-build.md and nix-instantiate.md. -->
+- A normal filesystem path, like `/home/meow/nixfiles/default.nix`
+  - Including a directory, like `/home/meow/nixfiles`, equivalent to above
+- A single lookup path, like `<nixpkgs>` or `<nixos>`
+- A URL to a tarball, like `https://github.com/NixOS/nixpkgs/archive/refs/heads/release-23.11.tar.gz`
+- A flakeref, introduced by the prefix `flake:`, like `flake:git+https://git.lix.systems/lix-project/lix`
+- A channel name, introduced by the prefix `channel:`, like `channel:nixpkgs-unstable`.
+  - This uses a hard-coded URL pattern and is *not* related to the subscribed channels managed by the [nix-channel](./nix-channel.md) command.

doc/manual/src/command-ref/nix-build.md

@@ -4,7 +4,7 @@
 
 # Synopsis
 
-`nix-build` [*paths…*]
+`nix-build` [*fileish…*]
   [`--arg` *name* *value*]
   [`--argstr` *name* *value*]
   [{`--attr` | `-A`} *attrPath*]
@@ -20,19 +20,55 @@ For documentation on the latter, run `nix build --help` or see `man nix3-build`.
 # Description
 
 The `nix-build` command builds the derivations described by the Nix
-expressions in *paths*. If the build succeeds, it places a symlink to
+expressions in each *fileish*. If the build succeeds, it places a symlink to
 the result in the current directory. The symlink is called `result`. If
 there are multiple Nix expressions, or the Nix expressions evaluate to
 multiple derivations, multiple sequentially numbered symlinks are
 created (`result`, `result-2`, and so on).
 
-If no *paths* are specified, then `nix-build` will use `default.nix` in
+If no *fileish* is specified, then `nix-build` will use `default.nix` in
 the current directory, if it exists.
 
-If an element of *paths* starts with `http://` or `https://`, it is
+## Fileish Syntax
-interpreted as the URL of a tarball that will be downloaded and unpacked
+
-to a temporary location. The tarball must include a single top-level
+A given *fileish* may take one of a few different forms, the first being a simple filesystem path, e.g. `nix-build /tmp/some-file.nix`.
-directory containing at least a file named `default.nix`.
+This has the same semantics as the [import builtin](../language/builtins.md#builtins-import), so specifying a directory is equivalent to specifying `default.nix` within that directory.
+It may also be a [search path](./env-common.html#env-NIX_PATH) (also known as a lookup path) like `<nixpkgs>`, which is convenient to use with `--attr`/`-A`:
+
+```console
+$ nix-build '<nixpkgs>' -A firefox
+```
+
+(Note the quotation marks around `<nixpkgs>`, which will be necessary in most Unix shells.)
+
+If a *fileish* starts with `http://` or `https://`, it is interpreted as the URL of a tarball which will be fetched and unpacked.
+Lix will then `import` the unpacked directory, so these tarballs must include at least single top-level directory with a file called `default.nix`
+For example, you could build from a specific version of Nixpkgs with something like:
+
+```console
+$ nix-build "https://github.com/NixOS/nixpkgs/archive/refs/heads/release-23.11.tar.gz" -A firefox
+```
+
+If a path starts with `flake:`, the rest of the argument is interpreted as a [flakeref](./new-cli/nix3-flake.html#flake-references) (see `nix flake --help` or `man nix3-flake`), which requires the "flakes" experimental feature to be enabled.
+Lix will fetch the flake, and then `import` its unpacked directory, so the flake must include a file called `default.nix`.
+For example, the flake analogue to the above `nix-build` commands are:
+
+```console
+$ nix-build flake:nixpkgs -A firefox
+$ nix-build flake:github:NixOS/nixpkgs/release-23.11 -A firefox
+```
+
+Finally, for legacy reasons, if a path starts with `channel:`, the rest of the argument is interpreted as the name of a channel to fetch from `https://nixos.org/channels/$CHANNEL_NAME/nixexprs.tar.xz`.
+This is a **hard coded URL** pattern and is *not* related the subscribed channels managed by the [nix-channel](./nix-channel.md) command.
+
+> **Note**: any of the special syntaxes may always be disambiguated by prefixing the path.
+> For example: a file in the current directory literally called `<nixpkgs>` can be addressed as `./<nixpkgs>`, to escape the special interpretation.
+
+In summary, a path argument may be one of:
+
+{{#include ./fileish-summary.md}}
+
+## Notes
 
 `nix-build` is essentially a wrapper around
 [`nix-instantiate`](nix-instantiate.md) (to translate a high-level Nix

doc/manual/src/command-ref/nix-instantiate.md

@@ -11,7 +11,7 @@
   [{`--attr`| `-A`} *attrPath*]
   [`--add-root` *path*]
   [`--expr` | `-E`]
-  *files…*
+  *fileish…*
 
 `nix-instantiate` `--find-file` *files…*
 
@@ -25,8 +25,11 @@ of the resulting store derivations are printed on standard output.
 
 [store derivation]: ../glossary.md#gloss-store-derivation
 
-If *files* is the character `-`, then a Nix expression will be read from
+If *fileish* is the character `-`, then a Nix expression will be read from standard input.
-standard input.
+Otherwise, each *fileish* is interpreted the same as with [nix-build](./nix-build.md#Path_Syntax).
+See that section for complete details (`nix-build --help`), but in summary, a path argument may be one of:
+
+{{#include ./fileish-summary.md}}
 
 # Options