[Form] Add documentation for DataMapperInterface

[B-Galati]

Hello,

I think it would be a really good idea to document this interface as it represents an awesome feature with forms. It has been documented on some blog posts like webmozart but that's all.

What do you think ? Is it voluntary ?
Thanks.

[ogizanagi]

Hi @B-Galati .

There is a pending PR about this long-time undocumented feature: See #6900

If you're more interested by this feature, you can also give a look to symfony/symfony#19367 ;)

[B-Galati]

Thanks @ogizanagi ! I looked for a similar issue/PR before opening this one but I missed them 😞

Anyway, thanks again. I close this issue as it is a duplicated.