Improve git.util.cwd docstring

This frames the reentrancy claim in terms of what is similar to and
different from contextlib.chdir (which we would just use, but it is
only available on Python 3.11 and later).

git/util.py

@@ -150,7 +150,10 @@ def wrapper(self: "Remote", *args: Any, **kwargs: Any) -> T:
 
 @contextlib.contextmanager
 def cwd(new_dir: PathLike) -> Generator[PathLike, None, None]:
-    """Context manager to temporarily change directory. Not reentrant."""
+    """Context manager to temporarily change directory.
+
+    This is similar to contextlib.chdir introduced in Python 3.11, but the context
+    manager object returned by a single call to this function is not reentrant."""
     old_dir = os.getcwd()
     os.chdir(new_dir)
     try: