Standard practices for incomplete/semi-experimental methods

[Tokazama]

It's pretty hard to figure out what's going on in Julia Base for newbies given all the undocumented internals. There are a lot of comments but those are poorly maintained and have variable quality. I propose we require documentation with some standard !!! warning Unstable API for all methods that are used throughout Julia Base but are not intended for public use.

This is related to #48819, but it's probably safe to add a documentation requirement to the contributors guide prior to any other bells and whistles.

[Tokazama]

#49298 is a good example where this has bit us in the ass. There appears to be two packages that use Base.unionlen although it is undocumented and not exported. Bike-shedding code is already obnoxious enough without worrying about people using stuff that is clearly not intended to be used outside of Base.

[adienes]

I think this is the public keyword now

of course there are plenty of things that could use more documentation besides, but at least now documentation is decoupled from API