Document the required private HTTPX2 hook

[adamtheturtle]

Issue

MockVWS deliberately patches HTTPX2's private sync and async transport selectors so it can intercept clients created before the mock starts. The narrow Pylint, Ruff, and Pyright suppressions did not explain why a public API could not replace that access.

HTTPX has the same private-only selector, and RESPX patches it in the same way, so HTTPX2 does not currently lag a public HTTPX extension point and no HTTPX2 parity issue is appropriate.

Change

Add a concise comment beside the suppressions linking to RESPX's permanent source revision and explaining the existing-client requirement.

Validation

• All configured pre-commit hooks
• All configured pre-push hooks, including Mypy, Pyright, verifytypes, Ty, Pyrefly, docs, manifest, and partition checks
• Full credential-free non-Docker suite: 1,941 passed, 849 skipped
• Docker-backed suite: 6 passed