explanatory note about macros SWIG and USER_API

CMakeLists.txt

@@ -79,6 +79,10 @@ option(BORNAGAIN_DEBUG_OPTIMIZATION "Build with debug optimization (gcc only)" O
 option(BORNAGAIN_TIDY "Invokes clang-tidy" OFF)
 option(ALGORITHM_DIAGNOSTIC "Let some algorithms set diagnostic variables" OFF)
 
+# note on two non-options:
+# macro SWIG is defined when code is processed by the tool Swig.
+# macros SWIG and USER_API are set by Doc/Doxygen/user/Doxyfile.in
+
 if(BORNAGAIN_TIDY AND BORNAGAIN_PYTHON)
     message(FATAL_ERROR "BORNAGAIN_TIDY is incompatible with BORNAGAIN_PYTHON")
 endif()