docs: clarify how value propagation works in CustomField

[vursen]

Fixes https://github.com/vaadin/docs/issues/2864

[russelljtdyer]

Please write some text introducing the section, "Value Type & Format". For instance, "The value type and format of the Custom Field can be done in Java or in Lit & React. The following two sections explain how you might do this given the programming language or approach you prefer."

[vursen]

Please write some text introducing the section, "Value Type & Format". For instance, "The value type and format of the Custom Field can be done in Java or in Lit & React. The following two sections explain how you might do this given the programming language or approach you prefer."

Thanks, added.