Help: Add release notes for updated generator expressions

Help/release/dev/genex-comma-separated.rst

@@ -0,0 +1,8 @@
+genex-comma-separated
+---------------------
+
+* The :manual:`generator expressions <cmake-generator-expressions(7)>`
+  ``C_COMPILER_ID``, ``CXX_COMPILER_ID``, ``CUDA_COMPILER_ID``,
+  ``Fortran_COMPILER_ID``, ``COMPILE_LANGUAGE``, ``COMPILE_LANG_AND_ID``, and
+  ``PLATFORM_ID`` learned to support matching one value from a comma-separated
+  list.