docs: tweaks

src/ModifierSigs.ml

@@ -83,7 +83,7 @@ sig
   *)
 
   val register_printer : ([ `NotFound of context option * Trie.bwd_path | `Shadow of context option * Trie.bwd_path * (data * tag) * (data * tag) | `Hook of context option * Trie.bwd_path * hook * (data, tag) Trie.t ] -> string option) -> unit
-  (** [register_printer f] registers a printer [p] via {!val:Printexc.register_printer} to turn unhandled internal effects into strings for the OCaml runtime system to display them. The functor {!module:Modifier.Make} always registers a simple printer to suggest using {!val:run}, but you can register new ones to override it. The registered printers are executed in reverse order; that is, the last registered printer is executed first. The return type of the printer [p] is [string option], where {!constructor:None} means the printer passes on the (unhandled) effect and lets the OCaml runtime system try the next printer or (if no more printers are available) use the default system message.
+  (** [register_printer f] registers a printer [p] via {!val:Printexc.register_printer} to turn unhandled internal effects into strings for the OCaml runtime system to display them. The functor {!module:Modifier.Make} always registers a simple printer to suggest using {!val:run}, but you can register new ones to override it. The return type of the printer [p] should return [Some s] where [s] is the resulting string, or [None] if it chooses not to convert a particular effect. The registered printers are tried in reverse order until one of them returns [Some s] for some [s]; that is, the last registered printer is tried first. Note that this function is a wrapper of {!val:Printexc.register_printer} and all the registered printers (via this function or {!val:Printexc.register_printer}) are put into the same list.
 
       The input type of the printer [p] is a variant representation of all internal effects used in this module. They correspond to the three effect triggers in {!module:Perform}. More precisely,
       - [`NotFound (ctx, prefix)] corresponds to the effect triggered by [Perform.not_found ctx prefix]; and