Help/cmake-cxxmodules: note Ubuntu 26.04 requirement for GCC `import std`

Help/manual/cmake-cxxmodules.7.rst

@@ -84,6 +84,13 @@ library combinations:
 * MSVC toolset 14.36 and newer (provided with Visual Studio 17.6 and newer)
 * GCC 15 and newer
 
+  .. note::
+
+    Ubuntu prior to 26.04 ships broken ``libstdc++.modules.json`` files.
+    See `Ubuntu issue 2141579`_.
+
+.. _`Ubuntu issue 2141579`: https://bugs.launchpad.net/ubuntu/+source/gcc-15/+bug/2141579
+
 The :variable:`CMAKE_CXX_COMPILER_IMPORT_STD` variable lists standard levels
 which have support for ``import std`` in the active C++ toolchain.