pgsql: Docs: improve description of psql's %R prompt escape sequence.

Docs: improve description of psql's %R prompt escape sequence.

Dilian Palauzov pointed out in bug #14201 that the docs failed to mention
the possibility of %R producing '(' due to an unmatched parenthesis.

He proposed just adding that in the same style as the other options were
listed; but it seemed to me that the sentence was already nearly
unintelligible, so I rewrote it a bit more extensively.

Report: <20160619121113(dot)5789(dot)68274(at)wrigleys(dot)postgresql(dot)org>

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/b9b925f83fbbed9ae7dd60d94dcd19eb6f6d2bcd

Modified Files
--------------
doc/src/sgml/ref/psql-ref.sgml | 36 ++++++++++++++++++++----------------
1 file changed, 20 insertions(+), 16 deletions(-)