Update ticks.callback documentation

docs/axes/labelling.md

@@ -17,11 +17,25 @@ Namespace: `options.scales[scaleId].title`, it defines options for the scale tit
 
 ## Creating Custom Tick Formats
 
-It is also common to want to change the tick marks to include information about the data type. For example, adding a dollar sign ('$'). To do this, you need to override the `ticks.callback` method in the axis configuration.
-In the following example, every label of the Y-axis would be displayed with a dollar sign at the front.
+It is also common to want to change the tick marks to include information about the data type. For example, adding a dollar sign ('$').
+To do this, you need to override the `ticks.callback` method in the axis configuration.
+
+The method receiver 3 arguments:
+
+* `value` - the tick value in the **internal data format** of the associated scale.
+* `index` - the tick index in the ticks array.
+* `ticks` - the array containing all of the [tick objects](../api/interfaces/tick).
+
+The call to the method is scoped to the scale. `this` inside the method is the scale object.
 
 If the callback returns `null` or `undefined` the associated grid line will be hidden.
 
+:::tip
+The [category axis](../axes/cartesian/category), which is the default x-axis for line and bar charts, uses the `index` as internal data format. For accessing the label, use `this.getLabelForValue(value)`. [API: getLabelForValue](../api/classes/scale.html#getlabelforvalue)
+:::
+
+In the following example, every label of the Y-axis would be displayed with a dollar sign at the front.
+
 ```javascript
 var chart = new Chart(ctx, {
     type: 'line',
@@ -41,4 +55,6 @@ var chart = new Chart(ctx, {
 });
 ```
 
-The third parameter passed to the callback function is an array of labels, but in the time scale, it is an array of `{label: string, major: boolean}` objects.
+Related samples:
+
+* [Tick configuration sample](../samples/scale-options/ticks)