Add a comment.

test/CMakeLists.txt

@@ -3,6 +3,7 @@
 include(CheckCXXSourceCompiles)
 set(CMAKE_REQUIRED_INCLUDES ${CMAKE_CURRENT_SOURCE_DIR}/..)
 
+# Writing a wide character to a character stream Writer is forbidden.
 check_cxx_source_compiles("
   #include \"format.cc\"
   int main() { fmt::Writer() << L'a'; }