More minor updates and copy-editing.

doc/src/sgml/ref/alter_database.sgml

doc/src/sgml/ref/alter_group.sgml

doc/src/sgml/ref/alter_table.sgml

doc/src/sgml/ref/alter_user.sgml

doc/src/sgml/ref/checkpoint.sgml

doc/src/sgml/ref/close.sgml

doc/src/sgml/ref/cluster.sgml

doc/src/sgml/ref/comment.sgml

doc/src/sgml/ref/copy.sgml

doc/src/sgml/ref/create_aggregate.sgml

doc/src/sgml/ref/create_domain.sgml

doc/src/sgml/ref/create_function.sgml

doc/src/sgml/ref/create_group.sgml

doc/src/sgml/ref/create_index.sgml

doc/src/sgml/ref/create_language.sgml

doc/src/sgml/ref/create_operator.sgml

doc/src/sgml/ref/create_rule.sgml

doc/src/sgml/ref/create_schema.sgml

doc/src/sgml/ref/create_table.sgml

doc/src/sgml/ref/create_table_as.sgml