docs: Add documentation for enums (#415)

* 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
Copy-Tag: eyJwIjoiLmdpdGh1Yi8uT3dsQm90LnlhbWwiLCJoIjoiMDA4MGY4MzBkZWMzN2MzMzg0MTU3MDgyYmNlMjc5ZTM3MDc5ZWE1OCJ9

* 🦉 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/language_v1/services/language_service/client.py

@@ -1067,7 +1067,7 @@ def sample_annotate_text():
         # Done; return the response.
         return response
 
-    def __enter__(self):
+    def __enter__(self) -> "LanguageServiceClient":
         return self
 
     def __exit__(self, type, value, traceback):