Code review for DOMAIN patch.

doc/src/sgml/catalogs.sgml

doc/src/sgml/ref/create_domain.sgml

doc/src/sgml/ref/drop_domain.sgml

doc/src/sgml/ref/psql-ref.sgml

src/backend/catalog/heap.c

src/backend/catalog/pg_aggregate.c

src/backend/catalog/pg_proc.c

src/backend/catalog/pg_type.c

src/backend/commands/creatinh.c

src/backend/commands/define.c

src/backend/commands/indexcmds.c

src/backend/commands/remove.c

src/backend/executor/nodeAgg.c

src/backend/nodes/copyfuncs.c

src/backend/nodes/equalfuncs.c

src/backend/nodes/makefuncs.c

src/backend/optimizer/path/indxpath.c

src/backend/optimizer/prep/preptlist.c

src/backend/parser/gram.y

src/backend/parser/parse_coerce.c