* [#not-implemented-hide:] will hide a message/enum/field from docs.
* [#not-implemented-warn:] will add a "Not implemented yet" warning to a
message/enum/field.
* [#v2-api-diff:<text>] will add a note indicating v2 API difference to
a message/enum/field.
* Switched title annotation to [#protodoc-title:<text>] for consistency.
Signed-off-by: Harvey Tuch <htuch@google.com>