pgsql: enable_constraint_exclusion => constraint_exclusion Also improve

Log Message:
-----------
enable_constraint_exclusion => constraint_exclusion

Also improve wording.

Modified Files:
--------------
pgsql/doc:
TODO (r1.1624 -> r1.1625)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1624&r2=1.1625)
pgsql/doc/src/FAQ:
TODO.html (r1.131 -> r1.132)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.131&r2=1.132)
pgsql/doc/src/sgml:
runtime.sgml (r1.346 -> r1.347)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.346&r2=1.347)
pgsql/src/backend/optimizer/path:
allpaths.c (r1.135 -> r1.136)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/allpaths.c.diff?r1=1.135&r2=1.136)
pgsql/src/backend/utils/misc:
guc.c (r1.285 -> r1.286)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.285&r2=1.286)
postgresql.conf.sample (r1.160 -> r1.161)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.160&r2=1.161)
pgsql/src/include/optimizer:
cost.h (r1.69 -> r1.70)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/optimizer/cost.h.diff?r1=1.69&r2=1.70)
pgsql/src/test/regress/expected:
rangefuncs.out (r1.12 -> r1.13)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/rangefuncs.out.diff?r1=1.12&r2=1.13)