docs: Add documentation for enums (googleapis#336)

* docs: Add documentation for enums

fix: Add context manager return types

chore: Update gapic-generator-python to v1.8.1
PiperOrigin-RevId: 503210727

Source-Link: googleapis/googleapis@a391fd1

Source-Link: https://github.com/googleapis/googleapis-gen/commit/0080f830dec37c3384157082bce279e37079ea58
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* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: gcf-owl-bot[bot]

google/cloud/container_v1/services/cluster_manager/client.py

@@ -4754,7 +4754,7 @@ def sample_list_usable_subnetworks():
         # Done; return the response.
         return response
 
-    def __enter__(self):
+    def __enter__(self) -> "ClusterManagerClient":
         return self
 
     def __exit__(self, type, value, traceback):