Updated zend.http.headers.rst to reflect header-specific methods

Updated `zend.http.headers.rst` to reflect the header-specific methods exposed by each of the different abstract base classes.

Also updated methods for:

- `Accept*`
- `RetryAfter`
- `SetCookie`

This is a follow up to #1125.

(I did my best to ensure that this PR is typo-free.)