RFC: fix #54664, make at-doc not change when REPL is loaded

[JeffBezanson]

This is a possible fix, by more fully separating the core docsystem from the presentation layer. @doc always returns a DocStr or vector of DocStrs, and a new REPL.@showdoc handles parsing markdown and filling in helpful text when no docstring is found.

This allows a new behavior: when you copy a docstring using @doc (@doc foo) bar (considered harmful), if @doc foo returns multiple items there is an error. Previously it would concatenate them into a single markdown document and use that as the new docstring, which seems a bit crazy to me.

This is breaking of course; for your consideration.

[StefanKarpinski]

Would suggest @docstr instead of @showdoc since show suggests printing rather than returning as string. Would be good to get a legal opinion on whether the current documentation requires the current behavior. Would be good to run PkgEval on this change to see how bad carnage is. Even if we technically can change this (not documented API), if the breakage is too significant, we may not want to anyway.