Unclear path substition rules

It is not clear how to specify the substitute syn::Paths required as argument.

A few things remain puzzling:

1. what is the relative base module needed to get a substitution match?
2. does it have to be absolute paths?
3. do bare types (so i.e. just an Ident) themselves suffice for matching?

let path = &ty.ty.path;
            // Don't generate a type if it was substituted - the target type might
            // not be in the type registry + our resolution already performs the substitution.
            if self.settings.substitutes.contains(dbg!(&path.segments)) {
                continue;
            }

is the relevant line, but it's not clear what path.segments contains and relative to what base.

Start at the substitution check using self.settings.substitutes.contains(...) and inspect what path.segments contains during matching. Trace the substitution behavior to determine the relative-module, absolute-path, and bare-identifier rules, then document those answers clearly; the work is done when users can specify substitute syn::Paths without this ambiguity.