Document the `have_http_status` matcher.

Outline the intended use cases for the `have_http_status` matcher:

- It is designed to work of "response" objects
- It should work with controller, request, and feature specs
- It allows checking against:
  - Specific numeric status codes
  - Any of the Rails provided symbolic names for status codes
  - General status code categories
REWORD: BASIC IMP DONE; YARD!!!