feat(libft.h): update ft_memcmp description

libft.h

@@ -129,7 +129,11 @@ int	ft_strncmp(const char *s1, const char *s2, size_t n);
 */
 void	*ft_memchr(const void *s, int c, size_t n);
 
-// TODO
+/*
+	The memcmp() function compares byte
+	string s1 against byte string s2.  
+	Both strings are assumed to be n bytes long.
+*/
 int	ft_memcmp(const void *s1, const void *s2, size_t n);
 
 // TODO