Minor cleanup in markup, especially in the Output section.

doc/src/sgml/ref/alter_table.sgml

doc/src/sgml/ref/alter_user.sgml

doc/src/sgml/ref/begin.sgml

doc/src/sgml/ref/close.sgml

doc/src/sgml/ref/cluster.sgml

doc/src/sgml/ref/commands.sgml

doc/src/sgml/ref/commit.sgml

doc/src/sgml/ref/copy.sgml

doc/src/sgml/ref/create_aggregate.sgml

doc/src/sgml/ref/create_database.sgml

doc/src/sgml/ref/create_function.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_sequence.sgml

doc/src/sgml/ref/create_table.sgml

doc/src/sgml/ref/create_trigger.sgml

doc/src/sgml/ref/create_type.sgml

doc/src/sgml/ref/create_user.sgml