Add module-level docstring to public HTTPX API

[enarciso2009]

This PR adds a module-level docstring to httpx/__init__.py.

The goal is to improve IDE autocompletion and inline documentation when exploring
the public HTTPX API. No functional behavior is changed.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.