To add pg_temp schema description to schemas documentation

 Good day.

 I've   found  that  the pg_temp schema alias is mentioned in the
description of the search_path variable, but  is   missing   from
the schemas documentation section.

 I  think it would be good to have that there, as that section is
mentioned as an extended reference for schemas.

 This  patch  adds a small paragraph about the temporary  schemas
to the schemas documentation section.