docs(ngChange): clarify when the ngChange expression is evaluated

Closes  angular#7866

Author: petebacondarwin

src/ng/directive/input.js

@@ -2201,9 +2201,15 @@ var ngModelDirective = function() {
  * The expression is evaluated immediately, unlike the JavaScript onchange event
  * which only triggers at the end of a change (usually, when the user leaves the
  * form element or presses the return key).
- * The expression is not evaluated when the value change is coming from the model
- * or when the input does not match the pattern defined for the input in ngPattern.
- * 
+ *
+ * The `ngChange` expression is only evaluated when a change in the input value causes
+ * a new value to be committed to the model.
+ *
+ * It will not be evaluated:
+ * * if the value returned from the `$parsers` transformation pipeline has not changed
+ * * if the input has continued to be invalid since the model will stay `null`
+ * * if the model is changed programmatically and not by a change to the input value
+ *
  *
  * Note, this directive requires `ngModel` to be present.
  *