pgsql: Add a warning about possible strange behavior of volatile

Log Message:
-----------
Add a warning about possible strange behavior of volatile functions
in cursors.  This has always been the case, but given the lack of user
complaints about it, I'm not going to bother back-patching this.

Modified Files:
--------------
    pgsql/doc/src/sgml/ref:
        declare.sgml (r1.46 -> r1.47)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/declare.sgml?r1=1.46&r2=1.47)