Comment in portal.h

Hi,

I noticed $SUBJECT while working on something else:

/*
* Where we store tuples for a held cursor or a PORTAL_ONE_RETURNING or
* PORTAL_UTIL_SELECT query. (A cursor held past the end of its
* transaction no longer has any active executor state.)
*/
Tuplestorestate *holdStore; /* store for holdable cursors */
MemoryContext holdContext; /* memory containing holdStore */

We do that for PORTAL_ONE_MOD_WITH as well, so the comment should be
"Where we store tuples for a held cursor or a PORTAL_ONE_RETURNING,
PORTAL_ONE_MOD_WITH, or PORTAL_UTIL_SELECT query.". Attached is a
patch for that.

Best regards,
Etsuro Fujita