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.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
array.sgml (r1.52 -> r1.52.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/array.sgml.diff?r1=1.52&r2=1.52.2.1)
datatype.sgml (r1.181 -> r1.181.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.181&r2=1.181.2.1)
func.sgml (r1.347.2.2 -> r1.347.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.347.2.2&r2=1.347.2.3)
libpq.sgml (r1.220 -> r1.220.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.220&r2=1.220.2.1)
plperl.sgml (r2.59 -> r2.59.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plperl.sgml.diff?r1=2.59&r2=2.59.2.1)
plpgsql.sgml (r1.101 -> r1.101.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.101&r2=1.101.2.1)
pltcl.sgml (r2.42 -> r2.42.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/pltcl.sgml.diff?r1=2.42&r2=2.42.2.1)
rowtypes.sgml (r2.6 -> r2.6.4.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/rowtypes.sgml.diff?r1=2.6&r2=2.6.4.1)
xfunc.sgml (r1.120 -> r1.120.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.120&r2=1.120.2.1)