docs($compile): add some detail about $onChanges

Closes #15604

src/ng/compile.js

@@ -128,7 +128,8 @@
  * * `$onChanges(changesObj)` - Called whenever one-way (`<`) or interpolation (`@`) bindings are updated. The
  *   `changesObj` is a hash whose keys are the names of the bound properties that have changed, and the values are an
  *   object of the form `{ currentValue, previousValue, isFirstChange() }`. Use this hook to trigger updates within a
- *   component such as cloning the bound value to prevent accidental mutation of the outer value.
+ *   component such as cloning the bound value to prevent accidental mutation of the outer value. Note that this will
+ *   also be called when your bindings are initialized. 
  * * `$doCheck()` - Called on each turn of the digest cycle. Provides an opportunity to detect and act on
  *   changes. Any actions that you wish to take in response to the changes that you detect must be
  *   invoked from this hook; implementing this has no effect on when `$onChanges` is called. For example, this hook