Add notes about session changes to migration guide.
Update docs for session changes.
AuthComponent::user() is no longer static.
Add docs for missing method.
Fix broken TOC entry.
Update request docs.

* Remove duplicated content, and make docs sectioned like other new docs.
Update titles to be less repetitive.
More tweaks.
More request docs improvements.
Remove versionadded tag.

The docs are almost entirely new so this isn't that relevant.
Update session handler docs to not be wrong.
Merge pull request #1361 from cakephp/3.0-session

3.0 session