Help: add a manpage for cxxmodule support

Help/index.rst

@@ -58,6 +58,7 @@ Reference Manuals
    /manual/cmake-commands.7
    /manual/cmake-compile-features.7
    /manual/cmake-configure-log.7
+   /manual/cmake-cxxmodules.7
    /manual/cmake-developer.7
    /manual/cmake-env-variables.7
    /manual/cmake-file-api.7

Help/manual/cmake-cxxmodules.7.rst

@@ -0,0 +1,55 @@
+.. cmake-manual-description: CMake C++ Modules Support Reference
+
+cmake-cxxmodules(7)
+*******************
+
+.. versionadded:: 3.28
+
+C++ 20 introduced the concept of "modules" to the language.  The design
+requires build systems to order compilations among each other to satisfy
+``import`` statements reliably.  CMake's implementation asks the compiler
+to scan source files for module dependencies during the build, collates
+scanning results to infer ordering constraints, and tells the build tool
+how to dynamically update the build graph.
+
+Compiler Support
+================
+
+Compilers which CMake natively supports module dependency scanning include:
+
+* MSVC toolset 14.34 and newer (provided with Visual Studio 17.4 and newer)
+* LLVM/Clang 16.0 and newer
+* GCC 14 (for the in-development branch, after 2023-09-20) and newer
+
+Generator Support
+=================
+
+The list of generators which support scanning sources for C++ modules include:
+
+- :generator:`Ninja`
+- :generator:`Ninja Multi-Config`
+- :generator:`Visual Studio 17 2022`
+
+Limitations
+-----------
+
+There are a number of known limitations of the current C++ module support in
+CMake.  This does not document known limitations or bugs in compilers as these
+can change over time.
+
+For all generators:
+
+- Header units are not supported.
+- No builtin support for ``import std;`` or other compiler-provided modules.
+
+For the Ninja Generators:
+
+- ``ninja`` 1.11 or newer is required.
+
+For the :ref:`Visual Studio Generators`:
+
+- Only Visual Studio 2022 and MSVC toolsets 14.34 (Visual Studio
+  17.4) and newer.
+- No support for exporting or installing BMI or module information.
+- No diagnosis of using modules provided by ``PRIVATE`` sources from
+  ``PUBLIC`` module sources.