better explanaton of `open` keyword

[xhudik]

Since open keyword is a new concept of Scala 3. It would desire a bit more info in: https://docs.scala-lang.org/scala3/book/domain-modeling-oop.html (section: Planning for Extension). This is a first place where a reader finds this term.

A possible source of inspiration:
https://medium.com/@knoldus/what-is-open-class-in-scala-3-14876e047fd4