message: docs: Add heading to the router writable properties deprecation
  The deprecation note is missing a header. This commit adds one.
  PR Close
diff: @@ -379,6 +379,8 @@ The injector no longer requires the Reflect polyfill, reducing application size
  <a id="router-writable-properties"></a>
  +### Public `Router` properties
  +
  None of the public properties of the `Router` are meant to be writeable.
  They should all be configured using other methods, all of which have been
  documented.
  @@ -404,6 +406,8 @@ The following options are deprecated in entirely:
  <a id="router-can-load"></a>
  +### `CanLoad` guards
  +
  `CanLoad` guards in the Router are deprecated in favor of `CanMatch`. These guards execute at the same time
  in the lifecycle of a navigation. A `CanMatch` guard which returns false will prevent the `Route` from being
  matched at all and also prevent loading the children of the `Route`. `CanMatch` guards can accomplish the same
  