remove the Channels section

this is a how-to guide which should not be in the reference manual.
it also refers to `nix-env`, which should not be the first thing readers
of the reference manual encounter, as it behaves very differently in
spirit from the rest of Nix.

slightly reword the documentation to be more concise and informative.

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

@@ -8,36 +8,41 @@
 
 # Description
 
-A Nix channel is a mechanism that allows you to automatically stay
-up-to-date with a set of pre-built Nix expressions. A Nix channel is
-just a URL that points to a place containing a set of Nix expressions.
+Channels are a mechanism for referencing remote Nix expressions and conveniently retrieving their latest version.
+For the list of official channels, visit <https://nixos.org/channels>.
 
-To see the list of official NixOS channels, visit
-<https://nixos.org/channels>.
+> **Note**
+>
+> The state of a subscribed channel is external to the Nix expressions relying on it.
+> This may limit reproducibility.
+>
+> Dependencies on other Nix expressions can be declared explicitly with:
+> - [`fetchurl`](@docroot@/language/builtins.md#builtins-fetchurl), [`fetchTarball`](@docroot@/language/builtins.md#builtins-fetchTarball), or [`fetchGit`](@docroot@/language/builtins.md#builtins-fetchGit) in Nix expressions
+> - the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I) in command line invocations
 
 This command has the following operations:
 
   - `--add` *url* \[*name*\]\
-    Adds a channel named *name* with URL *url* to the list of subscribed
-    channels. If *name* is omitted, it defaults to the last component of
-    *url*, with the suffixes `-stable` or `-unstable` removed.
+    Add a channel *name* located at *url* to the list of subscribed channels.
+    If *name* is omitted, default to the last component of *url*, with the suffixes `-stable` or `-unstable` removed.
+
+    > **Note**
+    >
+    > `--add` does not automatically perform an update.
+    > Use `--update` explicitly.
 
     A channel URL must point to a directory containing a file `nixexprs.tar.gz`.
     At the top level, that tarball must contain a single directory with a `default.nix` file that serves as the channel’s entry point.
 
   - `--remove` *name*\
-    Removes the channel named *name* from the list of subscribed
-    channels.
+    Remove the channel *name* from the list of subscribed channels.
 
   - `--list`\
-    Prints the names and URLs of all subscribed channels on standard
-    output.
+    Print the names and URLs of all subscribed channels on standard output.
 
   - `--update` \[*names*…\]\
-    Downloads the Nix expressions of all subscribed channels (or only
-    those included in *names* if specified) and makes them the default
-    for `nix-env` operations (by symlinking them from the directory
-    `~/.nix-defexpr`).
+    Download the Nix expressions of subscribed channels and create a new generation.
+    Update all channels if none is specified, and only those included in *names* otherwise.
 
   - `--list-generations`\
     Prints a list of all the current existing generations for the
@@ -49,13 +54,8 @@ This command has the following operations:
     ```
 
   - `--rollback` \[*generation*\]\
-    Reverts the previous call to `nix-channel
-                    --update`. Optionally, you can specify a specific channel generation
-    number to restore.
-
-Note that `--add` does not automatically perform an update.
-
-The list of subscribed channels is stored in `~/.nix-channels`.
+    Revert channels to the state before the last call to `nix-channel --update`.
+    Optionally, you can specify a specific channel *generation* number to restore.
 
 {{#include ./opt-common.md}}
 
@@ -69,23 +69,33 @@ The list of subscribed channels is stored in `~/.nix-channels`.
 
 # Examples
 
-To subscribe to the Nixpkgs channel and install the GNU Hello package:
+Subscribe to the Nixpkgs channel and run `hello` from the GNU Hello package:
 
 ```console
 $ nix-channel --add https://nixos.org/channels/nixpkgs-unstable
+$ nix-channel --list
+nixpkgs https://nixos.org/channels/nixpkgs
 $ nix-channel --update
-$ nix-env --install --attr nixpkgs.hello
+$ nix-shell -p hello --run hello
+hello
 ```
 
-You can revert channel updates using `--rollback`:
+Revert channel updates using `--rollback`:
 
 ```console
-$ nix-instantiate --eval --expr '(import <nixpkgs> {}).lib.version'
-"14.04.527.0e935f1"
+$ nix-instantiate --eval '<nixpkgs>' --attr lib.version
+"22.11pre296212.530a53dcbc9"
 
 $ nix-channel --rollback
 switching from generation 483 to 482
 
-$ nix-instantiate --eval --expr '(import <nixpkgs> {}).lib.version'
-"14.04.526.dbadfad"
+$ nix-instantiate --eval '<nixpkgs>' --attr lib.version
+"22.11pre281526.d0419badfad"
+```
+
+Remove a channel:
+
+```console
+$ nix-channel --remove nixpkgs
+$ nix-channel --list
 ```