Document once kwarg in event.listen and event.listen_for as being present in 0.9.4 instead of 0.9.3, testing and changelog agree with me

Removed ungrammatical apostrophes from documentation, replacing
"it's" with "its" where appropriate (but in a few cases with "it is"
when that read better).

While doing that, I also fixed a couple of minor typos etc.
as I noticed them.

and :func:`.event.listens_for`.  This is a convenience feature which
will wrap the given listener such that it is only invoked once.

methods, classes, builtins, functools.partial(), everything known so far
- use get_callable_argspec() within ColumnDefault._maybe_wrap_callable, re: #2979

level (e.g. on the :class:`.Mapper` or :class:`.ClassManager`
level, as opposed to on an individual mapped class, and also on
:class:`.Connection`) that also made use of internal argument conversion
(which is most within those categories) would fail to be removable.
fixes #2973

is done

concurrent ability to return connections from the pool means that the
"first_connect" event is now no longer synchronized either, thus leading
to dialect mis-configurations under even minimal concurrency situations.

system would cause a recursion overflow due to usage of inspect.getargspec()
on it in order to detect a legacy calling signature for certain events,
and apparently there's no way to do this with a partial object.  Instead
we skip the legacy check and assume the modern style; the check itself
now only occurs for the SessionEvents.after_bulk_update and
SessionEvents.after_bulk_delete events.  Those two events will require
the new signature style if assigned to a "partial" event listener.
[ticket:2905]