JLine 3: documentation

[SethTisue]

at minimum, we'll need a good PR description and release note

-Xnojline was dropped deprecated, the new -Xjline:... needs doc

stretch goal would be to add stuff to https://docs.scala-lang.org/overviews/repl/overview.html

[SethTisue]

we should call users' attention equally to things we actually did, and things that JLine 3 just gives us for free

[SethTisue]

PR description is now adequate for release. room for improvement, but removing "blocker" label

[SethTisue]

@eed3si9n suggested doc "somewhere more permanent like readme or scala-lang site, describing why/when JNA is needed"