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
Let's change to something like this:

> **Note:** Forms can have a maximum of 18 fields. If you have more than 18 fields in your form, you should break it down using lists or nested values. This limitation is the result of implementation details in Play and the fact that Scala case classes are [limited to 22 fields](https://issues.scala-lang.org/browse/SI-7099).