docs($compile): clarify link fn's controller argument

Also add "bindToController" to exampe directive definition object.

Closes #10815

Author: Narretz

src/ng/compile.js

@@ -76,6 +76,7 @@
  *       scope: false,
  *       controller: function($scope, $element, $attrs, $transclude, otherInjectables) { ... },
  *       controllerAs: 'stringIdentifier',
+ *       bindToController: false,
  *       require: 'siblingDirectiveName', // or // ['^parentDirectiveName', '?optionalDirectiveName', '?^optionalParent'],
  *       compile: function compile(tElement, tAttrs, transclude) {
  *         return {
@@ -395,9 +396,15 @@
  *   * `iAttrs` - instance attributes - Normalized list of attributes declared on this element shared
  *     between all directive linking functions.
  *
- *   * `controller` - a controller instance - A controller instance if at least one directive on the
- *     element defines a controller. The controller is shared among all the directives, which allows
- *     the directives to use the controllers as a communication channel.
+ *   * `controller` - the directive's required controller instance(s) - Instances are shared
+ *     among all directives, which allows the directives to use the controllers as a communication
+ *     channel. The exact value depends on the directive's `require` property:
+ *       * `string`: the controller instance
+ *       * `array`: array of controller instances
+ *       * no controller(s) required: `undefined`
+ *
+ *     If a required controller cannot be found, and it is optional, the instance is `null`,
+ *     otherwise the {@link error:$compile:ctreq Missing Required Controller} error is thrown.
  *
  *   * `transcludeFn` - A transclude linking function pre-bound to the correct transclusion scope.
  *     This is the same as the `$transclude`