Improve the annotation documentation
Improve the annotation documentation

Summary:
Improve the annotation documentation:
* Make Scope Annotations and Transitive Annotations subsections of Standard Annotations since they are standard annotations
* Move Thrift annotations to the beginning since it seems more logical to have them next to other generic annotations followed by language-specific ones
* Clarify that `cpp.ref` and `cpp.ref_type` are deprecated in favor of `cpp.Ref`.
* Use `thrift.Box` instead of deprecated `cpp.ref` in the example.
* Fix naming conventions in the example.
* member -> field
* Various wording tweaks.

Reviewed By: Mizuchi

Differential Revision: D40397295

fbshipit-source-id: b0b54654d5ca872590a133b4a0140d986bc64936

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift