Improve comment of DeallocateStmt->isall

This field is not used directly in the code, but it is important for
query jumbling to be able to make a difference between a named
DEALLOCATE and DEALLOCATE ALL (see bb45156f34).  This behavior is
tracked in the regression tests of pg_stat_statements, but the reason
why this field is important can be easily missed, as a recent discussion
has proved, so let's improve its comment to document the reason why it
needs to be around.

Wording has been suggested by Tom Lane

Discussion: https://postgr.es/m/Zih1ATt37YFda8_p@paquier.xyz

src/include/nodes/parsenodes.h

@@ -4070,7 +4070,12 @@ typedef struct DeallocateStmt
 	NodeTag		type;
 	/* The name of the plan to remove, NULL if DEALLOCATE ALL */
 	char	   *name pg_node_attr(query_jumble_ignore);
-	/* true if DEALLOCATE ALL */
+
+	/*
+	 * True if DEALLOCATE ALL.  This is redundant with "name == NULL", but we
+	 * make it a separate field so that exactly this condition (and not the
+	 * precise name) will be accounted for in query jumbling.
+	 */
 	bool		isall;
 	/* token location, or -1 if unknown */
 	ParseLoc	location pg_node_attr(query_jumble_location);