STYLE: document that if(COMMAND) works also for macros and functions

Alex

Source/cmIfCommand.h

@@ -137,7 +137,8 @@ public:
       "  if(variable1 OR variable2)\n"
       "True if either variable would be considered true individually.\n"
       "  if(COMMAND command-name)\n"
-      "True if the given name is a command that can be invoked.\n"
+      "True if the given name is a command, macro or function that can be "
+      "invoked.\n"
       "  if(EXISTS file-name)\n"
       "  if(EXISTS directory-name)\n"
       "True if the named file or directory exists.  "