Last round of reference page editing.

doc/src/sgml/ref/declare.sgml

doc/src/sgml/ref/drop_aggregate.sgml

doc/src/sgml/ref/drop_cast.sgml

doc/src/sgml/ref/drop_conversion.sgml

doc/src/sgml/ref/drop_database.sgml

doc/src/sgml/ref/drop_domain.sgml

doc/src/sgml/ref/drop_function.sgml

doc/src/sgml/ref/drop_group.sgml

doc/src/sgml/ref/drop_index.sgml

doc/src/sgml/ref/drop_language.sgml

doc/src/sgml/ref/drop_opclass.sgml

doc/src/sgml/ref/drop_operator.sgml

doc/src/sgml/ref/drop_rule.sgml

doc/src/sgml/ref/drop_schema.sgml

doc/src/sgml/ref/drop_sequence.sgml

doc/src/sgml/ref/drop_table.sgml

doc/src/sgml/ref/drop_trigger.sgml

doc/src/sgml/ref/drop_type.sgml

doc/src/sgml/ref/drop_user.sgml

doc/src/sgml/ref/drop_view.sgml