Put some kind of grammatical uniformity in the <refpurpose> lines.

doc/src/sgml/ref/set.sgml

doc/src/sgml/ref/set_constraints.sgml

doc/src/sgml/ref/set_session_auth.sgml

doc/src/sgml/ref/set_transaction.sgml

doc/src/sgml/ref/show.sgml

doc/src/sgml/ref/truncate.sgml

doc/src/sgml/ref/unlisten.sgml

doc/src/sgml/ref/update.sgml

doc/src/sgml/ref/vacuum.sgml

doc/src/sgml/ref/vacuumdb.sgml

doc/src/sgml/reference.sgml