use plainto_tsquery for PG match

use plainto_tsquery for PG match

The :meth:`.Operators.match` operator now uses ``plainto_tsquery()`` for
PostgreSQL full text search, rather than ``to_tsquery()``. The rationale
for this change is to provide better cross-compatibility with match on
other database backends.    Full support for all PostgreSQL full text
functions remains available through the use of :data:`.func` in
conjunction with :meth:`.Operators.bool_op` (an improved version of
:meth:`.Operators.op` for boolean operators).

Additional doc updates here apply to 1.4 so will backport these
out to a separate commit.

Fixes: #7086
Change-Id: I1946075daf5d9c558e85f73f1bf852604b3b1b8c