clarify "Enable an app for specific groups of users"

so it is easier to find, e.g. when you search for "restrict"

Signed-off-by: Alexander Müller <28591861+alexanderdd@users.noreply.github.com>

Author: alexanderdd

admin_manual/occ_command.rst

@@ -268,7 +268,7 @@ Enable an app regardless of the Nextcloud version requirement::
  sudo -E -u www-data php occ app:enable --force files_external
  files_external enabled
 
-Enable an app for specific groups of users::
+Enable an app for specific groups of users (i.e. restrict an app so only specific groups can see and use them)::
 
  sudo -E -u www-data php occ app:enable --groups admin --groups sales files_external
  files_external enabled for groups: admin, sales