Improved invalidScanf error message

lib/checkio.cpp

@@ -426,7 +426,12 @@ void CheckIO::invalidScanfError(const Token *tok, bool portability)
                     "    return 0;\n"
                     "}\n"
                     "\n"
-                    "To make it crash, type in more than 5 characters.");
+                    "Typing in 5 or more characters may make the program crash. The correct usage "
+                    "here is 'scanf(\"%4s\", c);', as the maximum field width does not include the "
+                    "terminating null byte.\n"
+                    "Source: http://linux.die.net/man/3/scanf\n"
+                    "Source: http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/libkern/stdio/scanf.c"
+                   );
 }
 
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