docs(rules): state what actually binds a host module's name, on both engines

rules/mcpp.toml

@@ -17,9 +17,18 @@
 # manager that already exists — and written in C++, so the build does not grow
 # a second language the way xmake's Lua rules or Bazel's Starlark do.
 #
-# The package name is load-bearing: mcpp registers a host module under the
-# dependency's `package.name`, so the name here IS the module name and must be
-# a legal C++ module name (no hyphens, no dots).
+# The package name and the module name are kept EQUAL, and that is what makes
+# this package work on every engine rather than on the newest one.
+#
+# mcpp < 2026.8.29.1 registers a host module under the dependency's
+# `package.name`, so on those engines the name here IS the module name. From
+# 2026.8.29.1 the module name is what `src/grpcgen.cppm` declares, and the
+# package name carries no C++ naming constraint at all.
+#
+# Keeping the two equal satisfies both. Renaming either one alone would build
+# under GCC and fail under Clang and MSVC on the older engines -- GCC finds the
+# BMI by the declared name through gcm.cache, while the other two are handed an
+# explicit `<name>=<bmi>` mapping built from the registered name.
 #
 # Versioned in lock-step with grpc and grpc-plugin: all three ship from one
 # tag, and .github/workflows/ci.yml checks that they never drift apart.
@@ -32,8 +41,12 @@ license     = "Apache-2.0"
 repo        = "https://github.com/mcpplibs/grpc-m"
 platforms   = ["linux", "macos", "windows"]
 
-# No dependencies on purpose. A host module is compiled alongside build.mcpp,
-# where only `std` and the bundled `mcpp` module exist — see the note at the
-# top of src/grpcgen.cppm for why this rule uses neither.
+# No dependencies, and from 2026.8.29.1 that is a CHOICE rather than a limit.
+# Before it, a host module was compiled alone and could import only `std` and
+# the bundled `mcpp` module; since it, a rule may declare its own
+# `[build-dependencies]` and import any of them marked `host-module = true`.
+#
+# This rule stays a leaf so that it keeps working on older engines, and because
+# it has nothing to reuse: what it does is compute a protoc command line.
 [targets.grpcgen]
 kind = "lib"