Update code comment to reference stringForValue()

Adding back the comment but revised it to refer to the stringForValue() method  in the IValueFormatter protocol. I know, I know, it should have been obvious but @philfi had to point it out to me.

Source/Charts/Formatters/IValueFormatter.swift

@@ -11,10 +11,10 @@
 
 import Foundation
 
-/// Interface that allows custom formatting of all values inside the chart before they are being drawn to the screen.
-///
-/// Simply create your own formatting class and let it implement ValueFormatter.
+/// Interface that allows custom formatting of all values inside the chart before they are drawn to the screen.
 ///
+/// Simply create your own formatting class and let it implement ValueFormatter. Then override the stringForValue()
+/// method and return whatever you want.
 
 @objc(IChartValueFormatter)
 public protocol IValueFormatter: class
@@ -24,12 +24,10 @@ public protocol IValueFormatter: class
     ///
     /// For performance reasons, avoid excessive calculations and memory allocations inside this method.
     ///
-    /// - returns: The formatted label ready for being drawn
+    /// - returns:                   The formatted label ready to be drawn
     ///
     /// - parameter value:           The value to be formatted
     ///
-    /// - parameter axis:            The entry the value belongs to - in e.g. BarChart, this is of class BarEntry
-    ///
     /// - parameter dataSetIndex:    The index of the DataSet the entry in focus belongs to
     ///
     /// - parameter viewPortHandler: provides information about the current chart state (scale, translation, ...)