Create table of Message-specific methods for the Message description

[alicejli]

Currently the c.g.c pages for Messages combine generated base class methods with Message-specific methods.

For instance for PhraseSet part of google.cloud.speech, the base class methods equals and hashcode() are at the same level as containsAnnotations and getKmsKeyName.

Ideally, as part of the Enum description at the top of the page, there would be a condensed table of Message-specific methods and the description of what the method does, similar to the tables on the Package overviews.