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