Update documentation for SET to include SCHEMA / NAMES syntax

Hi,

The attached patch updates the documentation for the SET statement to
include the special syntax for SCHEMA / NAMES in the synopsis. The
current documentation only mentions this special syntax under the
parameters, which is confusing and different from the usual practice.

We could consider also moving the description on the special syntax up
to the description instead of keeping it under parameters, but the
TIME ZONE variant is already explained under the parameters too so I
thought better to keep the current structure.

Thanks,
Álvaro Rodríguez