pgsql: docs for DROP ...

Log Message:
-----------

docs for DROP ... IF EXISTS for the following cases:
  language, tablespace, trigger, rule, opclass, function, aggregate. operator, and cast.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        drop_aggregate.sgml (r1.27 -> r1.28)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_aggregate.sgml.diff?r1=1.27&r2=1.28)
        drop_cast.sgml (r1.6 -> r1.7)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_cast.sgml.diff?r1=1.6&r2=1.7)
        drop_function.sgml (r1.30 -> r1.31)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_function.sgml.diff?r1=1.30&r2=1.31)
        drop_language.sgml (r1.21 -> r1.22)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_language.sgml.diff?r1=1.21&r2=1.22)
        drop_opclass.sgml (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_opclass.sgml.diff?r1=1.8&r2=1.9)
        drop_operator.sgml (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_operator.sgml.diff?r1=1.25&r2=1.26)
        drop_rule.sgml (r1.20 -> r1.21)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_rule.sgml.diff?r1=1.20&r2=1.21)
        drop_tablespace.sgml (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_tablespace.sgml.diff?r1=1.2&r2=1.3)
        drop_trigger.sgml (r1.19 -> r1.20)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/drop_trigger.sgml.diff?r1=1.19&r2=1.20)