Document user files of nix

doc/manual/src/command-ref/files/default-nix-expression.md

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+# Default Nix expression
+
+The source for the default [Nix expressions](@docroot@/language/index.md) used by [`nix-env`].
+
+- `~/.nix-defexpr`
+- `$XDG_STATE_HOME/nix/defexpr` if [`use-xdg-base-directories`] is set to `true`.
+
+It is loaded as follows:
+
+- If the default expression is a file, it is loaded as a Nix expression.
+- If the default expression is a directory containing a `default.nix` file, that `default.nix` file is loaded as a Nix expression.
+- If the default expression is a directory without a `default.nix` file, then its contents (both files and subdirectories) are loaded as Nix expressions.
+  The expressions are combined into a single attribute set, each expression under an attribute with the same name as the original file or subdirectory.
+  Subdirectories without a `default.nix` file are traversed recursively in search of more Nix expressions, but the names of these intermediate directories are not added to the attribute paths of the default Nix expression.
+
+Then, the resulting expression is interpreted like this:
+
+- If the expression is an attribute set, it is used as the default Nix expression.
+- If the expression is a function, an empty set is passed as argument and the return value is used as the default Nix expression.
+
+
+For example, if the default expression contains two files, `foo.nix` and `bar.nix`, then the default Nix expression will be equivalent to
+
+```nix
+{
+  foo = import ~/.nix-defexpr/foo.nix;
+  bar = import ~/.nix-defexpr/bar.nix;
+}
+```
+
+The file `manifest.nix` is always ignored.
+
+The command [`nix-channel`] places a symlink to the user's current [channels profile](@docroot@/command-ref/files/channels.md) in this directory.
+This makes all subscribed channels available as attributes in the default expression.
+
+## User channel link
+
+A symlink that ensures that [`nix-env`] can find your channels.
+
+- `~/.nix-defexpr/channels`
+- `$XDG_STATE_HOME/defexpr/channels` if [`use-xdg-base-directories`] is set to `true`.
+
+This symlink points to:
+
+- `$XDG_STATE_HOME/profiles/channels` for regular users
+- `$NIX_STATE_DIR/profiles/per-user/root/channels` for `root`
+
+In a multi-user installation, you may also have `~/.nix-defexpr/channels_root`, which links to the channels of the root user.[`nix-env`]: ../nix-env.md
+
+[`nix-env`]: @docroot@/command-ref/nix-env.md
+[`nix-channel`]: @docroot@/command-ref/nix-channel.md
+[`use-xdg-base-directories`]: @docroot@/command-ref/conf-file.md#conf-use-xdg-base-directories