Remove incorrect docs.

Session class no longer has these methods.
Update docs referencing old style flash messages.
Update login code.

It was wrong.
Set user to session.
Updated info about AuthComponent's method changes and other general info
Merge pull request #1436 from ADmad/3.0

Updated info about AuthComponent's method changes and other general info
Merge pull request #1435 from cakephp/session-de-dupe

Update docs for session changes and FlashComponent