reno includes release notes touched in a target release
even if they are added from past releases.
The recent release note cleanup I made as part of doc8 adoption
and some other cleanup touched release notes from past releases.
As a result, we see several number of release notes from past
release in the Pike release notes unexpectedly.
Also adds a warning note to the release notes howto page
Change-Id: I127f96896488e9992d9309d76055b4883f6a62f3
Closes-Bug: #1708560
6f1ecf2466