Clarify `wait(::Task)` documentation (#53722)
Small follow-up to https://github.com/JuliaLang/julia/pull/53685, since
having critical documentation for a specific method be only mentioned in
a catch-all docstring isn't quite as helpful as it could be (e.g. when
looking at inline documentation with LSP, where it happens to know that
the variable being passed is a `Task`). In addition, this also documents
that `wait` on a `Task` will throw if given `current_task`.
I've also split the function into a prototype and the actual
implementation, so that the same "more specific docs at the method
definition" transform can be applied for `GenericCondition` too.
