Improve explanation of NIX_PATH prefix syntax

The previous wording seemed to imply that the "channel:" syntax would resolve to
a github archive URL, which is not the case.

doc/manual/src/command-ref/env-common.md

@@ -28,17 +28,21 @@ Most Nix commands interpret the following environment variables:
     consist of a single top-level directory. For example, setting
     `NIX_PATH` to
 
-        nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixos-15.09.tar.gz
+        nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
 
-    tells Nix to download the latest revision in the Nixpkgs/NixOS 15.09
-    channel.
+    tells Nix to download and use the current contents of the
+    `master` branch in the `nixpkgs` repository.
 
-    A following shorthand can be used to refer to the official channels:
+    The URLs of the tarballs from the official nixos.org channels (see
+    [the manual for `nix-channel`](nix-channel.md)) can be abbreviated
+    as `channel:<channel-name>`.  For instance, the following two
+    values of `NIX_PATH` are equivalent:
 
-        nixpkgs=channel:nixos-15.09
+        nixpkgs=channel:nixos-21.05
+        nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
 
-    The search path can be extended using the `-I` option, which takes
-    precedence over `NIX_PATH`.
+    The Nix search path can also be extended using the `-I` option to
+    many Nix commands, which takes precedence over `NIX_PATH`.
 
   - `NIX_IGNORE_SYMLINK_STORE`\
     Normally, the Nix store directory (typically `/nix/store`) is not