Add JSBigInt description

Author: hoodmane

peps/pep-0818.rst

@@ -793,7 +793,11 @@ JavaScript object.
 
 ``IS_CALLABLE``
     Signals whether the ``typeof`` the JavaScript object is ``"function"``. If
-    present, used to implement ``__call__`` on the ``JSProxy``.
+    present, used to implement ``__call__`` on the ``JSProxy``. Any keyword
+    arguments will be put into a JavaScript object and passed as the last
+    argument to the JavaScript function. It will also be used to implement a
+    `new` method such that ``jscallable.new(*args, **kwargs)`` is shorthand for
+    ``Reflect.construct(jsfunction, *args, **kwargs)``.
 
 ``IS_ERROR``
     Signals whether the JavaScript object is an ``Error``. If so, the
@@ -945,7 +949,7 @@ The ``JSProxy`` base class has the following shape:
 
 .. code:: python
 
-    class JSProxy(metaclass=_JsProxyMetaClass):
+    class JSProxy(metaclass=_JSProxyMetaClass):
         def __eq__(self, other: JSProxy) -> bool:
             ...
         
@@ -1265,7 +1269,14 @@ This has the following properties:
 
 ``JSProxy``: This is ``type(run_js("({})"))``
 
-``JSBigInt``
+``JSBigInt``: This is a subclass of ``int`` that behaves in all ways identical
+to ``int`` except that it is guaranteed to be converted to a ``BigInt`` when
+passed to JavaScript. Otherwise, only ``int``s that are bigger than ``2**53``
+are converted to ``BigInt``, and smaller ``int``s are converted to ``number``.
+If the left operand on a binary op or the leftmost operand of `pow` is a
+``BigInt``, the operation returns a ``BigInt``. If the leftmost operand is an
+``int`` the return value will be an ``int``.
+
 
 
 The ``jstypes.code`` Module