Enable direct usage of --repl-init-script with Scala REPL >= 3.6.4-RC1

[Gedochao]

A follow-up to scala/scala3#21242

Meant to clear-up confusion like in https://contributors.scala-lang.org/t/scala-3-6-4-release-thread/7010/2

This adds some docs and improves the UX of --repl-init-script, enabling it to be used directly with the REPL, rather than after -- or with -O.

scala-cli repl -S 3.6.4-RC1 --repl-init-script 'println("Hello")'
# Hello
# Welcome to Scala 3.6.4-RC1 (23.0.1, Java OpenJDK 64-Bit Server VM).
# Type in expressions for evaluation. Or try :help.
#                                                                                                                  
# scala> 

cc @noti0na1 @bjornregnell