Document user files of nix

doc/manual/src/command-ref/files/channels.md

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+# Channels
+
+A directory containing symlinks to Nix channels, managed by [`nix-channel`].
+
+- `$NIX_STATE_DIR/profiles/per-user/root/channels` for `root`
+- `$XDG_STATE_HOME/nix/profiles/channels` for regular users
+
+[`nix-channel`] uses a [profile](@docroot@/command-ref/files/user-profiles.md) to store channels.
+This profile contains symlinks to the contents of those channels.
+
+## Subscribed channels
+
+The list of subscribed channels is stored in
+
+- `~/.nix-channels`
+- `$XDG_STATE_HOME/nix/channels` if [`use-xdg-base-directories`] is set to `true`
+
+in the following format:
+
+```
+<url> <name>
+...
+```
+
+[`nix-channel`]: @docroot@/command-ref/nix-channel.md
+[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories