Doc: reword discussion of asterisk after table names in FROM.

Doc: reword discussion of asterisk after table names in FROM.

The syntax "tablename *" has been obsolete for years, but we want to
retain it and its documentation for backward compatibility reasons.
However, the documentation wording was confusing and could be
understood to mean that "tablename *" is the same as "ONLY tablename".

Reported-by: Jochen Bandhauer <jochen.bandhauer@gmx.net>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/178125831604.1285960.8250607197280951685@wrigleys.postgresql.org