add stacklevel=2 to Client warnings

[trim21]

Summary

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.

see this rule: https://docs.astral.sh/ruff/rules/no-explicit-stacklevel/

without stacklevel=2, warnings show on lines in httpx module, not in user code, where Client is created

[trim21]

I also recommend httpx enable this ruff rule

[tomchristie]

Thanks for this. Now obsoleted both of these occurrences in version-1.0 branch.