docs: update urllib3 docstrings for v2 compatibility (#1903)

Updates docstrings in `google/auth/transport/urllib3.py` to reference
`urllib3.PoolManager` instead of the removed/private
`urllib3.request.RequestMethods`, ensuring compatibility with `urllib3`
v2 documentation standards.

NOTE: This PR supercedes an [older PR
#1290](#1290),
because some of the content or suggested changes in the older PR are
overtaken by events and/or been incorporated via [other recent
PRs](#1389).

---
*PR created automatically by Jules for task
[17649723998293325458](https://jules.google.com/task/17649723998293325458)
started by @chalmerlowe*

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Chalmer Lowe <[email protected]>

Author: google-labs-jules[bot]

google/auth/transport/urllib3.py

@@ -106,9 +106,8 @@ class Request(transport.Request):
         credentials.refresh(request)
 
     Args:
-        http (urllib3.request.RequestMethods): An instance of any urllib3
-            class that implements :class:`~urllib3.request.RequestMethods`,
-            usually :class:`urllib3.PoolManager`.
+        http (urllib3.PoolManager): An instance of a urllib3 class that implements
+            the request interface (e.g. :class:`urllib3.PoolManager`).
 
     .. automethod:: __call__
     """
@@ -209,7 +208,7 @@ class AuthorizedHttp(RequestMethods):  # type: ignore
         response = authed_http.request(
             'GET', 'https://www.googleapis.com/storage/v1/b')
 
-    This class implements :class:`urllib3.request.RequestMethods` and can be
+    This class implements the urllib3 request interface and can be
     used just like any other :class:`urllib3.PoolManager`.
 
     The underlying :meth:`urlopen` implementation handles adding the