- Added new user-space accessors for viewing transaction isolation

- Added new user-space accessors for viewing transaction isolation
levels; :meth:`.Connection.get_isolation_level`,
:attr:`.Connection.default_isolation_level`.
- enhance documentation inter-linkage between new accessors,
existing isolation_level parameters, as well as in
the dialect-level methods which should be fully covered
by Engine/Connection level APIs now.

(cherry picked from commit c3d898e8d06c7e549bb273fc8654f5d24fab2204)