runner: export pytest.CallInfo for typing purposes

The type cannot be constructed directly, but is exported for use in type
annotations, since it is reachable through existing public API.

This also documents `from_call` as public, because at least
pytest-forked uses it, so we must treat it as public already anyway.

doc/en/reference.rst

@@ -758,7 +758,7 @@ Full reference to objects accessible from :ref:`fixtures <fixture>` or :ref:`hoo
 CallInfo
 ~~~~~~~~
 
-.. autoclass:: _pytest.runner.CallInfo()
+.. autoclass:: pytest.CallInfo()
     :members: