[Docs] More details on custom form field controls guide

#### Bug, feature request, or proposal:
I'd like to request a review of the 'Creating a custom form field control' guide as it does not cover enough in regards to each of it properties and opts to just mentioning them.

#### What is the expected behaviour?
Common examples that are detailed on other approaches and usages.

#### What is the current behaviour?
Common examples that are not all detailed on other approaches or usages.

#### What is the use-case or motivation for changing an existing behaviour?
My biggest gripe that pushed me to make this ticket is that of the error state.

> Since we're not using an NgControl in this example, we don't need to do anything other than just set it to false.

Where as it should be like 'ngControl' which has sufficient methods of implementation

> It is likely you will want to implement ControlValueAccessor ...

#### Is there anything else we should know?
Others have had the same issue

https://stackoverflow.com/questions/52017000/angular-material-custom-reactive-form-control-with-error-state/52017769

Inizia leggendo la guida «Creating a custom form field control» e confronta le descrizioni delle sue proprietà con gli esempi richiesti e la discussione sullo stato di errore. Il lavoro è completo quando la guida spiega gli usi comuni delle proprietà, mostra approcci pertinenti e chiarisce la gestione dello stato di errore e il contesto di ControlValueAccessor.

Scritto dal modello di indicizzazione a partire dal testo della issue.