Added message to clarify that the event only fires when <canvas> is being used.

src/drawer.js

@@ -1234,6 +1234,8 @@ function drawTiles( drawer, lastDrawn ){
           /**
            * This event is fired just before the tile is drawn giving the application a chance to alter the image.
            *
+           * NOTE: This event is only fired when the drawer is using a <canvas>.
+           *
            * @event tile-drawing
            * @memberof OpenSeadragon.Viewer
            * @type {object}