Mention localectl in the docs

resources/default-config.kdl

@@ -17,7 +17,8 @@ input {
             // options "grp:win_space_toggle,compose:ralt,ctrl:nocaps"
 
             // If this section is empty, niri will fetch xkb settings
-            // from org.freedesktop.locale1.
+            // from org.freedesktop.locale1. You can control these using
+            // localectl set-x11-keymap.
         }
 
         // Enable numlock on startup, omitting this setting disables it.

wiki/Configuration:-Input.md

@@ -149,6 +149,27 @@ input {
 >
 > If the `xkb` section is empty (like it is by default), niri will fetch xkb settings from systemd-localed at `org.freedesktop.locale1` over D-Bus.
 > This way, for example, system installers can dynamically set the niri keyboard layout.
+> You can see this layout in `localectl` and change it with `localectl set-x11-keymap`, for example:
+>
+> ```sh
+> $ localectl set-x11-keymap "us" "" "colemak_dh_ortho" "compose:ralt,ctrl:nocaps"
+> $ localectl
+> System Locale: LANG=en_US.UTF-8
+>                LC_NUMERIC=ru_RU.UTF-8
+>                LC_TIME=ru_RU.UTF-8
+>                LC_MONETARY=ru_RU.UTF-8
+>                LC_PAPER=ru_RU.UTF-8
+>                LC_MEASUREMENT=ru_RU.UTF-8
+>     VC Keymap: us-colemak_dh_ortho
+>    X11 Layout: us
+>   X11 Variant: colemak_dh_ortho
+>   X11 Options: compose:ralt,ctrl:nocaps
+> ```
+>
+> By default, `localectl` will set the TTY keymap to the closest match of the XKB keymap.
+> You can prevent that with a `--no-convert` flag, for example: `localectl set-x11-keymap --no-convert "us,ru"`.
+>
+> These settings are picked up by some other programs too, like GDM.
 
 When using multiple layouts, niri can remember the current layout globally (the default) or per-window.
 You can control this with the `track-layout` option.