gapic: Add extra method/example comments for each REQUIRED field

Proto fields annotated as `REQUIRED` have documentation stating they are `Required`, but this doesn't appear anywhere in the client library documentation as the request message types are in a separate package with separate docs.

Each top-level request field annotated with `(google.api.field_behavior) = REQUIRED` should be:
* mentioned in the calling method's documentation as a required field
* mentioned in the calling method's example payload construction code as a required field (maybe even with a value initialized)

This will help make it easier for users to construct a valid request from just the client library documentation.

WDYT @tbpg @codyoss ?

Start by tracing how REQUIRED field annotations are used when generating calling-method documentation and example payload construction for Go client libraries. Verify that every top-level required request field is identified in both the method documentation and its example payload, with the generated client documentation providing enough information to construct a valid request.

Written by the indexing model from the issue text.