[Form] Add documentation for new tel and color types

[apetitpa]

https://symfony.com/blog/new-in-symfony-3-4-form-improvements#new-html5-types

[javiereguiluz]

After reading https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/color I wonder if the ColorType of Symfony follows the HTML5 spec regarding this:

The value of a element of type "color" is always a DOMString which contains a 7-character string specifying an RGB color in lower case hexadecimal notation. The value is never in any other form, and is never empty.

Setting the value to anything that isn't a valid, fully-opaque, RGB color in hexadecimal notation will result in the value being set to "#000000". In particular, you can't use CSS's standardized color names, or any CSS functions, to set the value.

[xabbuh]

Thank you @apetitpa.