[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.
Bah, nevermind - now I see that #4169 is here for the 2.5 features and that this is for 2.3.

So, obviously ignore (2), but I still think we should do 1, 3 and 4.