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: 3c987b385d35a8ba54e64dbdae7e9bf894a787d3@wrigleys.postgresql.org

50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb