add note maximum number of fileds limitation for a single tupple or mapp...

Note modification. maximum number of fileds limitation for a single tupple or mapping

I guess if there is written only about 22 limitation, newbies will be confused.
https://github.com/playframework/playframework/blob/2.3.x/framework/src/play/src/main/scala/play/api/data/Forms.scala
Tiny one: s/to a way how Forms.scala implements a mapping method/to the way form handling is implemented/