nixos/programs/bash: document that /etc/bash_completion.d is obsolete

fbd52a5e31/bash_completion (L2070-L2078)

nixos/modules/programs/bash/bash.nix

@@ -14,7 +14,10 @@ let
   bashCompletion = optionalString cfg.enableCompletion ''
     # Check whether we're running a version of Bash that has support for
     # programmable completion. If we do, enable all modules installed in
-    # the system (and user profile).
+    # the system and user profile in obsolete /etc/bash_completion.d/
+    # directories. Bash loads completions in all
+    # $XDG_DATA_DIRS/share/bash-completion/completions/
+    # on demand, so they do not need to be sourced here.
     if shopt -q progcomp &>/dev/null; then
       . "${pkgs.bash-completion}/etc/profile.d/bash_completion.sh"
       nullglobStatus=$(shopt -p nullglob)