Fix a pile of API doc errors in Database/
Fix a bunch more API doc errors.
Mark many of the Database internals as such.

Marking these classes as internal as they are subject to change for the
first few versions of 3.0. Until we get all the potential issues sorted
I don't want to make any backwards compatibility promises on these
classes.
Update class namespace.
Merge pull request #3723 from cakephp/database-api-docs

3.0 - Database api docs