Initial work on the documentation.
Made transaction.managed a no-op and deprecated it.

enter_transaction_management() was nearly always followed by managed().

In three places it wasn't, but they will all be refactored eventually.
The "forced" keyword argument avoids introducing behavior changes until
then.

This is fully backwards-compatible, except, of course, for managed
itself.
Added an API to control database-level autocommit.

Dropped uses_autocommit feature.
wip