docs($compile): clarify scope types and controllerAs

Closes #12537
Closes #12758

Author: Narretz

src/ng/compile.js

@@ -150,9 +150,10 @@
  *
  * * **falsy:** No scope will be created for the directive. The directive will use its parent's scope.
  *
- * * **`true`:** A new scope will be created for the directive's element. If multiple directives on the
- * same element request a new scope, only one new scope is created. The new scope rule does not apply
- * for the root of the template since the root of the template always gets a new scope.
+ * * **`true`:** A new child scope that prototypically inherits from its parent will be created for
+ * the directive's element. If multiple directives on the same element request a new scope,
+ * only one new scope is created. The new scope rule does not apply for the root of the template
+ * since the root of the template always gets a new scope.
  *
  * * **`{...}` (an object hash):** A new "isolate" scope is created for the directive's element. The
  * 'isolate' scope differs from normal scope in that it does not prototypically inherit from its parent
@@ -257,9 +258,10 @@
  *
  * #### `controllerAs`
  * Identifier name for a reference to the controller in the directive's scope.
- * This allows the controller to be referenced from the directive template. The directive
- * needs to define a scope for this configuration to be used. Useful in the case when
- * directive is used as component.
+ * This allows the controller to be referenced from the directive template. This is especially
+ * useful when a directive is used as component, i.e. with an `isolate` scope. It's also possible
+ * to use it in a directive without an `isolate` / `new` scope, but you need to be aware that the
+ * `controllerAs` reference might overwrite a property that already exists on the parent scope.
  *
  *
  * #### `restrict`