docs: add a couple of httpx2 classes to nitpick exceptions (#4948)

Fixes:
sphinx.errors.SphinxWarning:
  /opentelemetry-python-contrib/instrumentation/opentelemetry-instrumentation-httpx/src/opentelemetry/instrumentation/httpx/__init__.py:
  docstring of opentelemetry.instrumentation.httpx.AsyncOpenTelemetryTransportHttpx2:1:py:class reference target not found: httpx2.AsyncBaseTransport