Doc: Add OldSample to v2.rst

OldSample is still referenced in controllers/v2.py. In order for samples to be
generated in the documentation (thanks to the sample() class method), it must
also be referenced in doc/source/webapi/v2.rst.

Change-Id: Ie5bfe3aa1bc552630dad56eab3b2f921eb2b7690
Merge "Doc: Add OldSample to v2.rst"