On resource pages and others than wikitext content model a system message should be transcluded, before formatted content starts.
- On Lua pages there is already a documentation head configurable.
On resource pages .css .js .json a system message clearyourcache is present.
- enWP hijacked that for Editnotice transclusion etc.
Purpose
Display any guidance, link to project pages, explain data structure or whatever.
It might be switched depending on user language, content type, existence of documentation snippets etc. by template programming, one system message would fit all needs. Could cover scribunto as well, since different appearance.