Update compatibility.md with documentation of exceptions differences

[GlenTiki]

Summary

This change just documents the high level differences in exceptions exposed from `httpx` vs `requests`.

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]

Yes okay - thank you!