Added Canvas#toBuffer() docs

lib/canvas.js

@@ -174,6 +174,13 @@ Canvas.prototype.createPNGStream = function(){
   return new PNGStream(this);
 };
 
+/**
+ * Return a `Buffer` instance consisting of the PNG image data.
+ *
+ * @return {Buffer}
+ * @api public
+ */
+
 Canvas.prototype.toBuffer = function(){
   var buf;
   this.streamPNG(function(err, chunk, len){