pgsql: Update documentation for backslashes to mention escape string

Log Message:
-----------
Update documentation for backslashes to mention escape string syntax
more, and standard_conforming_strings less, because in the future non-E
strings will not treat backslashes specially.

Also use E'' strings where backslashes are used in examples. (The
existing examples would have drawn warnings.)

Backpatch to 8.2.X.

Modified Files:
--------------
pgsql/doc/src/sgml:
array.sgml (r1.52 -> r1.53)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/array.sgml.diff?r1=1.52&r2=1.53)
datatype.sgml (r1.187 -> r1.188)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.187&r2=1.188)
func.sgml (r1.354 -> r1.355)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.354&r2=1.355)
libpq.sgml (r1.221 -> r1.222)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.221&r2=1.222)
plperl.sgml (r2.59 -> r2.60)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml.diff?r1=2.59&r2=2.60)
plpgsql.sgml (r1.102 -> r1.103)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.102&r2=1.103)
pltcl.sgml (r2.42 -> r2.43)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pltcl.sgml.diff?r1=2.42&r2=2.43)
rowtypes.sgml (r2.6 -> r2.7)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/rowtypes.sgml.diff?r1=2.6&r2=2.7)
xfunc.sgml (r1.121 -> r1.122)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.121&r2=1.122)