Help/IMPORTED_CXX_MODULES_CONFIG: add docs

Help/manual/cmake-properties.7.rst

@@ -257,6 +257,7 @@ Properties on Targets
    /prop_tgt/IMPORTED_CXX_MODULES_COMPILE_DEFINITIONS
    /prop_tgt/IMPORTED_CXX_MODULES_COMPILE_FEATURES
    /prop_tgt/IMPORTED_CXX_MODULES_COMPILE_OPTIONS
+   /prop_tgt/IMPORTED_CXX_MODULES_CONFIG
    /prop_tgt/IMPORTED_CXX_MODULES_INCLUDE_DIRECTORIES
    /prop_tgt/IMPORTED_CXX_MODULES_LINK_LIBRARIES
    /prop_tgt/IMPORTED_GLOBAL

Help/prop_tgt/IMPORTED_CXX_MODULES_CONFIG.rst

@@ -0,0 +1,16 @@
+IMPORTED_CXX_MODULES_<CONFIG>
+-----------------------------
+
+.. versionadded:: 3.28
+
+A list of C++ module specifications available with the target for
+configuration ``<CONFIG>``.  Each item in the list is of the form
+``<NAME>=<INTERFACE>[,<BMI>]*`` where ``<NAME>`` is the name of the module,
+``<INTERFACE>`` is its module interface unit, and any number of available
+``<BMI>`` files are provided.
+
+.. note::
+
+   CMake currently does not use the ``<BMI>`` files as there is limited
+   support for determining whether an existing ``<BMI>`` file is suitable for
+   a given importer of the module it represents.