[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

まず「Creating a custom form field control」ガイドを読み、そのプロパティの説明を、要求された例およびエラー状態に関する説明と比較してください。ガイドが一般的なプロパティの使用方法を説明し、関連するアプローチを示し、エラー状態の処理と ControlValueAccessor のコンテキストを明確にしていれば完了です。

索引モデルが issue の本文から書いたものです。