More cleanup of Diagnostics sections.

doc/src/sgml/ref/alter_group.sgml

doc/src/sgml/ref/alter_language.sgml

doc/src/sgml/ref/alter_schema.sgml

doc/src/sgml/ref/cluster.sgml

doc/src/sgml/ref/clusterdb.sgml

doc/src/sgml/ref/commit.sgml

doc/src/sgml/ref/create_database.sgml

doc/src/sgml/ref/create_sequence.sgml

doc/src/sgml/ref/create_view.sgml

doc/src/sgml/ref/createdb.sgml

doc/src/sgml/ref/createuser.sgml

doc/src/sgml/ref/declare.sgml

doc/src/sgml/ref/delete.sgml

doc/src/sgml/ref/drop_database.sgml

doc/src/sgml/ref/drop_user.sgml

doc/src/sgml/ref/dropdb.sgml

doc/src/sgml/ref/dropuser.sgml

doc/src/sgml/ref/end.sgml

doc/src/sgml/ref/explain.sgml

doc/src/sgml/ref/fetch.sgml