function adjustDescendantCount(sId, iDiff) {
		mNodeById[sId].DescendantCount += iDiff;
		sId = mNodeById[sId].MANAGER_ID;
		if (sId) {
			adjustDescendantCount(sId, iDiff);
		}
	}
Adjust the DescendantCount of the node with given ID (and all of its ancestors) by the
given difference.

@param {string} sId - A node ID
@param {number} iDiff - Some difference