Help: Document when <LANG>_COMPILER_ID genex learned to support multiple ids

It was added in CMake 3.15.

Help/manual/cmake-generator-expressions.7.rst

@@ -1053,6 +1053,10 @@ related to most of the expressions in this sub-section.
   ``1`` if CMake's compiler id of the C compiler matches any one
   of the entries in ``compiler_ids``, otherwise ``0``.
 
+  .. versionchanged:: 3.15
+    Multiple ``compiler_ids`` can be specified.
+    CMake 3.14 and earlier only accepted a single compiler ID.
+
 .. genex:: $<CXX_COMPILER_ID>
 
   CMake's compiler id of the CXX compiler used.
@@ -1063,6 +1067,10 @@ related to most of the expressions in this sub-section.
   ``1`` if CMake's compiler id of the CXX compiler matches any one
   of the entries in ``compiler_ids``, otherwise ``0``.
 
+  .. versionchanged:: 3.15
+    Multiple ``compiler_ids`` can be specified.
+    CMake 3.14 and earlier only accepted a single compiler ID.
+
 .. genex:: $<CUDA_COMPILER_ID>
 
   .. versionadded:: 3.15
@@ -1115,6 +1123,10 @@ related to most of the expressions in this sub-section.
   ``1`` if CMake's compiler id of the Fortran compiler matches any one
   of the entries in ``compiler_ids``, otherwise ``0``.
 
+  .. versionchanged:: 3.15
+    Multiple ``compiler_ids`` can be specified.
+    CMake 3.14 and earlier only accepted a single compiler ID.
+
 .. genex:: $<HIP_COMPILER_ID>
 
   .. versionadded:: 3.21