docs: add documentation for documentsnapshot class (#263)

* docs: add documentation for documentsnapshot class

* chore: drop superseded docs fix

See PR #278.

Co-authored-by: Tres Seaver <tseaver@palladion.com>
Co-authored-by: Christopher Wilcox <crwilcox@google.com>

google/cloud/firestore_v1/document.py

@@ -455,7 +455,7 @@ def on_snapshot(self, callback: Callable) -> Watch:
         provided callback is run on the snapshot.
 
         Args:
-            callback(Callable[[:class:`~google.cloud.firestore.document.DocumentSnapshot`], NoneType]):
+            callback(Callable[[:class:`~google.cloud.firestore_v1.base_document.DocumentSnapshot`], NoneType]):
                 a callback to run when a change occurs
 
         Example: