Tweak comment

typing/typedecl.ml

@@ -1948,15 +1948,17 @@ let update_decls_jkind env decls =
    - if -rectypes is not used, we only allow cycles in the type graph
      if they go through an object or polymorphic variant type *)
 
-(* We only allow recursion of unboxed product types through boxing, otherwise
-   the type is uninhabitable and usually also infinite-size.
+(* We only allow recursion in unboxed product types to occur through boxes,
+   otherwise the type is uninhabitable and usually also infinite-size.
+   See [typing-layouts-unboxed-records/recursive.ml].
 
-   Because `check_well_founded` already ruled out recursion without through
-   structural types (unboxed tuples and aliases), we just look for a cycle in
-   paths.
+   Because `check_well_founded` already ruled out recursion through structural
+   types (for this check, we care about unboxed tuples and aliases), we just
+   look for a cycle in nominal unboxed types ([@@unboxed] types and unboxed
+   records), tracking the set of seen paths.
 
    An alternative implementation could have introduced layout variables (for
-   this check only), and find infinite-size types via the occurs check. Such an
+   this check only), finding infinite-size types via the occurs check. Such an
    implementation might be more principled, accepting finite-size recursive
    types like [type t = #{ t : t }]. The current implementation gives more
    informative error traces, and leaves the flexiblity to switch to a less