pgsql: Removes or minimizes some documentation mentions of backward

Log Message:
-----------
Removes or minimizes some documentation mentions of backward
compatibility for release 7.2 and earlier. I have not altered any
mentions of release 7.3 or later. The release notes were not modified,
so the changes are still documented, just not in the main docs.

Modified Files:
--------------
pgsql/doc/src/sgml:
array.sgml (r1.48 -> r1.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/array.sgml.diff?r1=1.48&r2=1.49)
backup.sgml (r2.79 -> r2.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.79&r2=2.80)
config.sgml (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.54&r2=1.55)
datatype.sgml (r1.166 -> r1.167)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datatype.sgml.diff?r1=1.166&r2=1.167)
datetime.sgml (r2.48 -> r2.49)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/datetime.sgml.diff?r1=2.48&r2=2.49)
ddl.sgml (r1.55 -> r1.56)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ddl.sgml.diff?r1=1.55&r2=1.56)
func.sgml (r1.313 -> r1.314)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.313&r2=1.314)
libpq.sgml (r1.206 -> r1.207)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.206&r2=1.207)
lobj.sgml (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/lobj.sgml.diff?r1=1.39&r2=1.40)
maintenance.sgml (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/maintenance.sgml.diff?r1=1.54&r2=1.55)
mvcc.sgml (r2.55 -> r2.56)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/mvcc.sgml.diff?r1=2.55&r2=2.56)
rules.sgml (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/rules.sgml.diff?r1=1.44&r2=1.45)
storage.sgml (r1.9 -> r1.10)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml.diff?r1=1.9&r2=1.10)
xfunc.sgml (r1.111 -> r1.112)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.111&r2=1.112)