ENH: clean up docs some

Source/cmAddDefinitionsCommand.h

@@ -69,7 +69,7 @@ public:
     {
     return
       "ADD_DEFINITIONS(-DFOO -DBAR ...)\n"
-      "Add -D define flags to command line for  environments.";
+      "Add flags to command line of C and C++ compiles.  This command can be used to add any flag to a compile line, but the -D flag is accepted most C/C++ compilers.  Other flags may not be as portable.";
     }
   
   cmTypeMacro(cmAddDefinitionsCommand, cmCommand);