Document the state of scripting/shebang lines for Scala 3

[rmgk]

I could not find a document regarding the use of shebang lines #!/some/executable in Scala 3 sources.
Experimentation seems to show that scalac correctly ignores those lines, but as far as I can tell that is not documented.

In particular, the fine people over at scalafmt rightfully complained that the feature request to have scalafmt support such lines is not valid without official documentation scalameta/scalafmt#3267