and modernize the engines chapter a bit

reflection now updates the PKC in place.
- support the use case of the empty PrimaryKeyConstraint in order to specify
constraint options; the columns marked as primary_key=True will now be gathered
into the columns collection, rather than being ignored. [ticket:2910]
- add validation such that column specification should only take place
in the PrimaryKeyConstraint directly, or by using primary_key=True flags;
if both are present, they have to match exactly, otherwise the condition is
assumed to be ambiguous, and a warning is emitted; the old behavior of
using the PKC columns only is maintained.

events including auto-invalidation, which is useful both for tests here as well as
detecting failure conditions within the "reset" or "close" cases.
- rename the argument for PoolEvents.reset() to dbapi_connection and connection_record
to be consistent with everything else.
- add new documentation sections on invalidation, including auto-invalidation
and the invalidation process within the pool.
- add _ConnectionFairy and _ConnectionRecord to the pool documentation.  Establish
docs for common _ConnectionFairy/_ConnectionRecord methods and accessors and
have PoolEvents docs refer to _ConnectionRecord,
since it is passed to all events.  Rename a few _ConnectionFairy methods that are actually
private to pool such as _checkout(), _checkin() and _checkout_existing(); there should not
be any external code calling these

More fixes for cross references and reducing warnings (3rd wave)

more flexible ways to set up bound parameters and return types;
in particular, a :func:`.text` can now be turned into a full
FROM-object, embeddable in other statements as an alias or CTE
using the new method :meth:`.TextClause.columns`.
[ticket:2877]