pgsql-server: Add note pointing out the difference in semantics between

Log Message:
-----------
Add note pointing out the difference in semantics between Oracle and
plpgsql EXCEPTION blocks.

Modified Files:
--------------
pgsql-server/doc/src/sgml:
plpgsql.sgml (r1.44 -> r1.45)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/plpgsql.sgml.diff?r1=1.44&r2=1.45)