SI-6898 Document AnyVal box and unbox implemention by BoxesRunTime

* Added `@boxRunTimeDoc@` and `@unboxRunTimeDoc@` tokens to `AnyVals`
* Doc comments refer to [BoxesRunTime.java](https://github.com/scala/scala/blob/master/src/library/scala/runtime/BoxesRunTime.java) in the Scala repo
* No comment for `Unit`