docs($http): improve description of caching

Included changes:

* Point out that only GET & JSONP requests are cached.
* Explain that the URL+search params are used as cache keys (headers not considered).
* Add note about cache-control headers on response not affecting Angular caching.
* Mention `$httpProvider.defaults.cache` (in addition to `$http.defaults.cache`).
* Clear up how `defaults.cache` and `config.cache` are taken into account for determining the
  caching behavior for each request.

Fixes #11101
Closes #13003