Fix Issue5416 - explain negative value for count in bytes object replace.

Objects/bytesobject.c

@@ -2150,7 +2150,8 @@ PyDoc_STRVAR(replace__doc__,
 \n\
 Return a copy of B with all occurrences of subsection\n\
 old replaced by new.  If the optional argument count is\n\
-given, only the first count occurrences are replaced.");
+positive, only the first count occurrences are replaced. A\n\
+negative value of count replaces all occurences");
 
 static PyObject *
 bytes_replace(PyBytesObject *self, PyObject *args)