[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.
Sorry, now that I read this note more closely, I'm not sure I agree (or probably more likely, I'm not sure it's phrased correctly). It *sounds* like you can set `error_bubbling` to `true` on the top-level form and then all the errors of the child fields will bubble up to it. But I don't think that's the case - it's set on a field, and then that field's error bubble to the form.

If I'm right, how about something like this:

>
If you enable the `error_bubbling` option on a field, calling ``getErrors()`` on the parent
form will include errors from that field. However, there is no way to determine which
field an error was originally attached to.

Thanks! And no worries about the delay - I'm catching up my self. And nice work with the above changes.