docs(guide/production): add more info on debugInfoEnabled()

Add bullet mentioning how placeholder comments are affected.

Closes angular#16155

Author: oBusk

docs/content/guide/production.ngdoc

@@ -20,6 +20,9 @@ and adds CSS classes to data-bound elements:
 CSS class are attached to the corresponding element. These scope references can then be accessed via
 `element.scope()` and `element.isolateScope()`.
 
+- Placeholder comments for structural directives will contain information about what directive
+and binding caused the placeholder. E.g. `<!-- ngIf: shouldShow() -->`.
+
 Tools like [Protractor](https://github.com/angular/protractor) and
 [Batarang](https://github.com/angular/angularjs-batarang) need this information to run,
 but you can disable this in production for a significant performance boost with: