Fix up the documentation nav for Spanner. (#4054)

docs/spanner/api-reference.rst

@@ -0,0 +1,33 @@
+API Reference
+=============
+
+The following classes and methods constitute the Spanner client.
+Most likely, you will be interacting almost exclusively with these:
+
+.. toctree::
+    :maxdepth: 1
+
+    client-api
+    instance-api
+    database-api
+    session-api
+    keyset-api
+    snapshot-api
+    batch-api
+    transaction-api
+    streamed-api
+
+
+The classes and methods above depend on the following, lower-level
+classes and methods. Documentation for these is provided for completion,
+and some advanced use cases may wish to interact with these directly:
+
+.. toctree::
+    :maxdepth: 1
+
+    gapic/v1/api
+    gapic/v1/types
+    gapic/v1/admin_database_api
+    gapic/v1/admin_database_types
+    gapic/v1/admin_instance_api
+    gapic/v1/admin_instance_types

docs/spanner/database-usage.rst

@@ -1,5 +1,5 @@
-Database Admin API
-==================
+Database Admin
+==============
 
 After creating a :class:`~google.cloud.spanner.instance.Instance`, you can
 interact with individual databases for that instance.

docs/spanner/instance-usage.rst

@@ -1,5 +1,5 @@
-Instance Admin API
-==================
+Instance Admin
+==============
 
 After creating a :class:`~google.cloud.spanner.client.Client`, you can
 interact with individual instances for a project.

docs/spanner/usage.rst

@@ -12,23 +12,7 @@ Spanner
   snapshot-usage
   transaction-usage
   advanced-session-pool-topics
-
-  client-api
-  instance-api
-  database-api
-  session-api
-  keyset-api
-  snapshot-api
-  batch-api
-  transaction-api
-  streamed-api
-
-  gapic/v1/api
-  gapic/v1/types
-  gapic/v1/admin_database_api
-  gapic/v1/admin_database_types
-  gapic/v1/admin_instance_api
-  gapic/v1/admin_instance_types
+  api-reference
 
 API requests are sent to the `Cloud Spanner`_ API via RPC over
 HTTP/2.  In order to support this, we'll rely on `gRPC`_.