pgsql: Doc: improve description of trim() and related functions.

Doc: improve description of trim() and related functions.

Per bug #14441 from Mark Pether, the documentation could be misread,
mainly because some of the examples failed to show what happens with
a multicharacter "characters to trim" string.  Also, while the text
description in most of these entries was fairly clear that the
"characters" argument is a set of characters not a substring to match,
some of them used variant wording that was a bit less clear.
trim() itself suffered from both deficiencies and was thus pretty
misinterpretable.

Also fix failure to explain which of LEADING/TRAILING/BOTH is the
default.

Discussion: https://postgr.es/m/20161130011710.6539.53657@wrigleys.postgresql.org

Branch
------
REL9_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/3e65fd045a924f4c4cfaa0f73857db28558afd11

Modified Files
--------------
doc/src/sgml/func.sgml | 27 ++++++++++++++-------------
1 file changed, 14 insertions(+), 13 deletions(-)