Improve docs for how to properly add container injection into a class that extends FormBase

In general we cannot inject anything for those base classes, because this would break a hell lot of implementations out there

Yup.

I believe we should trait-ify the remaining service wrappers in FormBase and be done with it.

The traits are neeto, but for instance StringTranslationTrait allows me to take a separate step to setStringTranslation() in my factory method, but then getNumberOfPlurals() ignores injected services anyway. That's out of scope here, but yah.

Also the other traits on FormBase have the same problem: If there's no injected service they use \Drupal when they should really complain.

So then the basic idea of FormBase is a little broken. All it gives you is the knowledge that you should implement FormInterface and then holds your hand if you did a bad job of dependency injection.

As it stands, it's a very poor example of how to implement ContainerInjectionInterface, because the router is going to hand you a container, which you ignore and then infer from \Drupal because it works.

In that case, let's document that in the docblock.

It's a doc change so 8.1.x.

I saw my own typo, so I fixed it, and then changed the docblock a bit. Here's the new one, easier to read here than in the patch:

/**
 * Provides a base class for forms.
 *
 * This class exists as a mid-point between dependency injection through
 * ContainerInjectionInterface, and a less-structured use of traits which
 * default to using the \Drupal accessor for service discovery.
 *
 * To properly inject services, override create() and use the setters provided
 * by the traits to inject the needed services.
 *
 * @code
 * public static function create($container) {
 *   $form = new static();
 *   // In this example we only need string translation so we use the
 *   // setStringTranslation() method provided by StringTranslationTrait.
 *   $form->setStringTranslation($container->get('string_translation'));
 *   return $form;
 * }
 * @endcode
 *
 * Alternately, you could not use FormBase, implement FormInterface, use only
 * the traits you actually need, and inject the services in a container
 * injection pattern you define.
 *
 * @ingroup form_api
 *
 * @see \Drupal\Core\DependencyInjection\ContainerInjectionInterface
 */

Setting back to RTBC after spurious testbot fail.

Changes per #15.