Revise the description of the max_resource_portals_per_transaction guc

[airfan1994]

The description of description in 6X_STABLE in same as max_resource_queues, which is very confusing.

{
                {"max_resource_queues", PGC_POSTMASTER, RESOURCES_MGM,
                        gettext_noop("Maximum number of resource queues."),
                        NULL
                },
                &MaxResourceQueues,
                9, 0, INT_MAX,
                NULL, NULL, NULL
        },

        {
                {"max_resource_portals_per_transaction", PGC_POSTMASTER, RESOURCES_MGM,
                        gettext_noop("Maximum number of resource queues."),
                        NULL
                },
                &MaxResourcePortalsPerXact,
                64, 0, INT_MAX,
                NULL, NULL, NULL
        },

I open a MR to Revise the description of the max_resource_portals_per_transaction guc. Please hava a look at this.