Documentation fix-up: "its" vs. "it's"

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.

a connection invalidation could occur within an already critical section
like a connection.close(); ultimately, these conditions are caused
by the change in :ticket:`2907`, in that the "reset on return" feature
calls out to the Connection/Transaction in order to handle it, where
"disconnect detection" might be caught.  However, it's possible that
the more recent change in :ticket:`2985` made it more likely for this
to be seen as the "connection invalidate" operation is much quicker,
as the issue is more reproducible on 0.9.4 than 0.9.3.

Checks are now added within any section that
an invalidate might occur to halt further disallowed operations
on the invalidated connection.  This includes two fixes both at the
engine level and at the pool level.   While the issue was observed
with highly concurrent gevent cases, it could in theory occur in
any kind of scenario where a disconnect occurs within the connection
close operation.
fixes #3043
ref #2985
ref #2907

- add some defensive checks during an invalidate situation:
1. _ConnectionRecord.invalidate might be called twice within finalize_fairy
if the _reset() raises an invalidate condition, invalidates, raises and then
goes to invalidate the CR.  so check for this.
2. similarly within Conneciton, anytime we do handle_dbapi_error(), we might become invalidated.
so a following finally must check self.__invalid before dealing with the connection
any futher.

Found using: https://github.com/intgr/topy

1. make sure pool._invalidate() sets the timestamp up before
invalidating the target connection.  we can otherwise show how the
conn.invalidate() + pool._invalidate() can lead to an extra connection
being made.
2. to help with that, soften up the check on connection.invalidate()
when connection is already closed. a warning is fine here
3. add a mutex to test_max_overflow() when we connect, because the way
we're using mock depends on an iterator, that needs to be synchronized

implementation allows an event handler to redefine the specific mechanics
by which an arbitrary dialect invokes execute() or executemany() on a
DBAPI cursor.  The new events, at this point semi-public and experimental,
are in support of some upcoming transaction-related extensions.

after one or more :class:`.Connection` objects have been created
(such as by an orm :class:`.Session` or via explicit connect)
and the listener will pick up events from those connections.
Previously, performance concerns pushed the event transfer from
:class:`.Engine` to  :class:`.Connection` at init-time only, but
we've inlined a bunch of conditional checks to make this possible
without any additional function calls. fixes #2978

within userland engine.dispose(); as some SQLA tests already failed when the replace step
was removed, due to those conns still being referenced, it's likely this will
create surprises for all those users that incorrectly use dispose()
and it's not really worth dealing with.  This doesn't affect the change
we made for ref: #2985.

recycles the connection pool when a "disconnect" condition is detected;
instead of discarding the pool and explicitly closing out connections,
the pool is retained and a "generational" timestamp is updated to
reflect the current time, thereby causing all existing connections
to be recycled when they are next checked out.   This greatly simplifies
the recycle process, removes the need for "waking up" connect attempts
waiting on the old pool and eliminates the race condition that many
immediately-discarded "pool" objects could be created during the
recycle operation. fixes #2985

emitted for the "_cursor_execute()" method of :class:`.Connection`;
this is the "quick" executor that is used for things like
when a sequence is executed ahead of an INSERT statement, as well as
for dialect startup checks like unicode returns, charset, etc.
the :meth:`.ConnectionEvents.before_cursor_execute` event was already
invoked here.  The "executemany" flag is now always set to False
here, as this event always corresponds to a single execution.
Previously the flag could be True if we were acting on behalf of
an executemany INSERT statement.