example-setups: Rename a private function

It returns BW patterns instead of EL patterns
README: add a link to the source file

The public functions are documented in there.  Brings that doc a bit
closer.
Allow match-spec for buffers with no filetype

You can now treat buffers without a filetype, like all other filetypes,
by using the key '!' in the match-setup.  Think of it as NOT.
Restore ability to override a fallback mode with an empty list

It got broken in c77614928dcaeb1beb577b498d01948d1d9517d6

You can now again override the fallback match-specification completely
with a filetype specific entry.  Checking an empty list (the default
return value if nothing is found) did of course not prevent a lookup in
the fallback record.