Separate / distinguish API docs for different API versions. (#6306)

Closes #6303.

tasks/docs/index.rst

@@ -1,11 +1,26 @@
 .. include:: /../tasks/README.rst
 
-Api Reference
+API Reference
 -------------
+
+This package includes clients for multiple versions of the Tasks
+API. By default, you will get ``v2beta3``, the latest version.
+
 .. toctree::
     :maxdepth: 2
 
-    gapic/v2beta2/api
-    gapic/v2beta2/types
     gapic/v2beta3/api
     gapic/v2beta3/types
+
+The previous beta release, spelled ``v2beta2``, is provided to continue to
+support code previously written against them. In order to use ththem, you
+will want to import from e.g.  ``google.cloud.tasks_v2beta2`` in lieu of
+``google.cloud.tasks`` (or the equivalent ``google.cloud.tasks_v2beta3``).
+
+An API and type reference is provided the this beta also:
+
+.. toctree::
+    :maxdepth: 2
+
+    gapic/v2beta2/api
+    gapic/v2beta2/types