Documentation: Validation and `valid` expressions
In an internet search I found the Github issue that proposed the `valid` key and looked into the code to ensure it is there.

I used `valid` to ensure that a file offset is 16-byte aligned. (Btw, if there is a way of doing it in an imperative manner, please tell me.)

Is the lacking documentation intended, because experimental feature, or could people just try to PR addtions to the documentation?

At the moment, there are five specific expression keywords used in ValidationSpec.scala : `min`, `max`, `eq`, `any-of`, `expr`.