Document the Annotation REST resources

On http://platform.xwiki.org/xwiki/bin/view/Features/XWikiRESTfulAPI is documented the main REST resources. However we have added Annotation REST resources and they are not documented. This needs to be added. The location where to add them needs to be decided.

Ideally the new REST resources should be documented in the Annotation extension (i.e. on http://extensions.xwiki.org/xwiki/bin/view/Extension/Annotations+Application) and linked from http://platform.xwiki.org/xwiki/bin/view/Features/XWikiRESTfulAPI