mconfig: improve consistency of comments.

mconfig

@@ -32,9 +32,9 @@ SANITIZEOPTS=-fsanitize=address,undefined
 #SANITIZEOPTS=-fsanitize=address,undefined
 
 # MacOS: use g++ (which may alias clang++):
+# Cannot use -fno-rtti: apparently prevents exception handling from working properly.
 #CXX=g++
 #CXXOPTS=-std=c++11 -Os -Wall -flto
 #LDFLAGS=-flto
 #BUILD_SHUTDOWN=no
 #SANITIZEOPTS=-fsanitize=address,undefined
-#  Note: -fno-rtti seems to break exception handling.