docs($compile): clarify scope types and controllerAs

Closes #12537
Closes #12758

Author: Narretz

Diff for: src/ng/compile.js

@@ -146,9 +146,12 @@
  * and other directives used in the directive's template will also be excluded from execution.
  *
  * #### `scope`
- * **If set to `true`,** then a new scope will be created for this directive. 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.
+ * **If not defined, or if set to a `falsy` value**, the directive will use the parent scope.
+ *
+ * **If set to `true`,** then a new child scope will be created for this directive that prototypically
+ * inherits from the parent scope. 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.
  *
  * **If set to `{}` (object hash),** then a new "isolate" scope is created. The 'isolate' scope differs from
  * normal scope in that it does not prototypically inherit from the parent scope. This is useful
@@ -238,9 +241,10 @@
  *
  * #### `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 a new scope, but you need to be aware that the
+ * `controllerAs` reference might overwrite a property that already exists on the scope.
  *
  *
  * #### `restrict`