nixos/xserver: Document the layouts sed expression
nixos/xserver: Document the layouts sed expression

This was brought up by @0xABAB in #26984 by the following comment:

https://github.com/NixOS/nixpkgs/pull/26984#discussion_r125146405

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@redmoonstudios.org>

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