Add documentation on context format

Author: etimberg

docs/axes/styling.md

@@ -21,6 +21,15 @@ The grid line configuration is nested under the scale configuration in the `grid
 | `offsetGridLines` | `boolean` | `false` | If true, grid lines will be shifted to be between labels. This is set to `true` for a bar chart by default.
 | `z` | `number` | `0` | z-index of gridline layer. Values <= 0 are drawn under datasets, > 0 on top.
 
+For function arguments, the function is passed a context object that is of the form:
+
+```javscript
+{
+    scale: // Scale object
+    tick: // Tick object
+}
+```
+
 ## Tick Configuration
 The tick configuration is nested under the scale configuration in the `ticks` key. It defines options for the tick marks that are generated by the axis.