Document empty tuple syntax (#4313)

Fixes #4211.

Author: emmatyping

docs/source/builtin_types.rst

@@ -13,6 +13,8 @@ Type                Description
 ``bytes``           8-bit string
 ``object``          an arbitrary object (``object`` is the common base class)
 ``List[str]``       list of ``str`` objects
+``Tuple[int, int]`` tuple of two ``int``s (``Tuple[()]`` is the empty tuple)
+``Tuple[int, ...]`` tuple of an arbitrary number of ``int`` objects
 ``Dict[str, int]``  dictionary from ``str`` keys to ``int`` values
 ``Iterable[int]``   iterable object containing ints
 ``Sequence[bool]``  sequence of booleans

docs/source/cheat_sheet.rst

@@ -33,6 +33,9 @@ Built-in types
    x = [1] # type: List[int]
    x = set([6, 7]) # type: Set[int]
 
+   # Empty Tuple types are a bit special
+   x = ()  # type: Tuple[()]
+
    # For mappings, we need the types of both keys and values.
    x = dict(field=2.0) # type: Dict[str, float]
 

docs/source/cheat_sheet_py3.rst

@@ -34,6 +34,9 @@ Built-in types
    x = [1]  # type: List[int]
    x = {6, 7}  # type: Set[int]
 
+   # Empty Tuple types are a bit special
+   x = ()  # type: Tuple[()]
+
    # For mappings, we need the types of both keys and values.
    x = {'field': 2.0}  # type: Dict[str, float]