Compose and display method information in Diviner

Summary:
Ref T988. As mentioned elsewhere, one broad goal of this iteration is to reduce the amount of boilerplate documentation we need to write. For example:

• I want to set @group by default in most cases.
• I want to inherit things like @param and @return by default.
• I want to inherit method documentation by default.
• I want to inherit @task information.

This implements most of the method inheritance stuff.

This looks super gross, but I believe we now compose all of the information of interest at display time and can work on rendering it sensibly in the near future.

Test Plan: {F56790}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran, sascha-egerer

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6849