doc: list both [cm_kind] match sites in narrowing reference

The Melange paragraph claimed the only mode-aware code was in
[Lib_file_deps.deps_of_entry_modules], but [need_impl_deps_of]
(inside [Module_compilation.lib_deps_for_module]) also matches on
[cm_kind] and explicitly names [Melange _] in the pattern. The
behaviour there is "Cmx reads trans-dep impl ocamldep; everything
else does not" — [Melange _] is in the same bucket as
[Ocaml (Cmi | Cmo)], not its own — but the doc oversimplified to
the point of inaccuracy.

Rewrite as a numbered list naming both [cm_kind] match sites and
clarifying that the second site's distinction is Cmx-vs-rest, not
OCaml-vs-Melange.

Replies to #14732 review comment r3304097840 (Copilot).

Signed-off-by: Robin Bate Boerop <me@robinbb.com>

Author: robinbb

doc/dev/per-module-narrowing.md

@@ -564,10 +564,17 @@ Three categories of fallback to the glob:
    silently drop the leaf library's `.cmi` from its compile rule.
 
 `Melange` consumer compiles run the same narrowing pipeline as `Ocaml`.
-The `can_filter` check is mode-agnostic; the only mode-specific code
-is in `Lib_file_deps.deps_of_entry_modules`, which emits per-module
-`.cmj` deps in addition to `.cmi` when `cm_kind = Melange Cmj`,
-mirroring the broad-dep path's `groups_for_cm_kind`.
+The `can_filter` check is mode-agnostic. Two code paths inside the
+pipeline match on `cm_kind`:
+
+1. `Lib_file_deps.deps_of_entry_modules` emits per-module `.cmj` in
+   addition to `.cmi` when `cm_kind = Melange Cmj`, mirroring the
+   broad-dep path's `groups_for_cm_kind`.
+2. `need_impl_deps_of` (in `lib_deps_for_module`) reads a trans-dep's
+   `.ml`-side ocamldep only for `cm_kind = Ocaml Cmx` (cross-module
+   inlining input). `Ocaml (Cmi | Cmo)` and `Melange _` are handled
+   symmetrically — neither reads impl deps. The distinction is
+   Cmx-vs-rest, not OCaml-vs-Melange.
 
 ## Cost characteristics