Sphinx provides the versionadded syntax for indicating version related
changes in documentation, the OpenStack doc theme is designed to handle
this and render it nicely. This patch switches our inconsistent
references to API version related change in the API ref to use this
syntax so that it standardised through out the docs, and is more obvious
in the documentation itself.
Change-Id: Ied6f22deecebd71042d2230b81e057543db3d3f4