Better documentation for how a resourceref works (#96)

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/cc @rambleraptor

plugins/modules/gcp_pubsub_subscription.py

@@ -75,10 +75,14 @@ options:
     suboptions:
       push_endpoint:
         description:
-        - A URL locating the endpoint to which messages should be pushed.
-        - For example, a Webhook endpoint might use "U(https://example.com/push".)
-        required: true
-      attributes:
+            - A reference to a Topic resource.
+            - 'This field represents a link to a Topic resource in GCP. It can be specified in
+              two ways. You can add `register: name-of-resource` to a gcp_pubsub_topic task and
+              then set this topic field to "{{ name-of-resource }}" Alternatively, you can set
+              this topic to a dictionary with the name key where the value is the name of your
+              Topic.'
+        required: false
+    push_config:
         description:
         - Endpoint configuration attributes.
         - Every endpoint has a set of API supported attributes that can be used to