Small clarification in docstring of dict.update(): the positional argument is not required.

Objects/dictobject.c

@@ -1973,9 +1973,9 @@ PyDoc_STRVAR(popitem__doc__,
 2-tuple; but raise KeyError if D is empty.");
 
 PyDoc_STRVAR(update__doc__,
-"D.update(E, **F) -> None.  Update D from dict/iterable E and F.\n"
-"If E has a .keys() method, does:     for k in E: D[k] = E[k]\n\
-If E lacks .keys() method, does:     for (k, v) in E: D[k] = v\n\
+"D.update([E, ]**F) -> None.  Update D from dict/iterable E and F.\n"
+"If E present and has a .keys() method, does:     for k in E: D[k] = E[k]\n\
+If E present and lacks .keys() method, does:     for (k, v) in E: D[k] = v\n\
 In either case, this is followed by: for k in F: D[k] = F[k]");
 
 PyDoc_STRVAR(fromkeys__doc__,