pgsql: Add documentation about the inability of plpsql to use parameter

Log Message:
-----------
Add documentation about the inability of plpsql to use parameter names
that are the same as column names used in the function.

Modified Files:
--------------
pgsql/doc/src/sgml:
plpgsql.sgml (r1.81 -> r1.82)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/plpgsql.sgml.diff?r1=1.81&r2=1.82)