- some doc reorg

- some doc reorg
- change engine.Connection to _connection_cls so sphinx doesn't get upset
- globally add "." to all :class:`Foo`
- start naming sections that are mostly docstrings "API Documentation - blah blah"
- move some ad-hoc docstrings into "API" sections, there is some inconsistency here
and it may be that we just have to leave it that way
- add "internals" rsts to core, orm, I'm not super thrilled how these look but they are
targeted by some of the public api docs, users typically become aware of these anyway