Docs: Add httpx.Proxy to api.md

[matusvalo]

Summary

Documentation of httpx does not mention the parameters of httpx.Proxy(). They can be important e.g. in the case of custom ssl_context. This PR documents them in the API Reference.

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.

[lovelydinosaur]

Thanks.