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

[noahdietz]

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 ?

[codyoss]

Maybe something like this in the example?

FooRequest{
    // Parent: Required
}