glib-compile-resources: Clarify --help output for --sourcedir argument
glib-compile-resources: Clarify --help output for --sourcedir argument

Make the --help output more consistent with the man page, making it more
obvious that --sourcedir only applies to the files referenced in FILE,
not to the location of FILE itself.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1406

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib