[Components][Form] describe how to access form errors

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

The ``getErrors()`` and ``getErrorsAsString()`` methods haven't been documented before. This pull request adds a short description for them. This forms as basis for the solution of #3660.
I think this note is a bit more abstract than I'd like. Here are the main points I'd like to say:

1) Calling `getErrors()` on the main form object only returns global form errors
2) Calling `getErrors(true)` will return *all* of the form objects, but there's no way to know which field each is attached to
3) Calling `$form['firstName']->getErrors()` will return you errors for only the `firstName` field
4) Calling `getErrorsAsString()` is easy for debugging.

There are a lot of questions about form errors on stackoverflow etc. and don't think we have details anywhere currently. So I'd like to fully explain how to do what you want. The only unfortunate thing (I think there was an issue/PR about this) is that `getErrors(true)` doesn't return any way (as far as a I can see) to know which error is attached to which field.