Clarify description of registries input

Author: aeisenberg

init/action.yml

@@ -15,6 +15,8 @@ inputs:
     required: false
   registries:
     description: |
+      Use this input only when you need to download CodeQL packages from another instance of GitHub. If you only need to download packages from this GitHub instance, use the token input instead.
+
       A YAML string that defines the list of GitHub container registries to use for downloading packs. The string is in the following form (the | is required on the first line):
 
       registries: |