docs: Improve documentation of \pset without arguments.

The syntax summary previously failed to clarify that the first
argument is also optional.  The textual description did mention it,
but all the way at the bottom.  It fits better with the command
overview, so move it there, and fix the summary also.

Back-patch to 9.4 where \pset without arguments was supported.

Dilip Kumar, reviewed by Fabien Coelho