Update text in rationale describing conversions of primitive types

Author: hoodmane

peps/pep-0818.rst

@@ -110,12 +110,14 @@ JavaScript has the following immutable types: ``string``, ``undefined``,
 ``boolean``, ``number`` and ``bigint``. It also has the special value ``null``.
 
 Of these, ``string`` and ``boolean`` directly correspond to ``str`` and
-``bool``. ``number`` and ``bigint`` awkwardly correspond to ``float`` and
-``int``. ``undefined`` is the default value for a missing argument so it
+``bool``. We convert a ``number`` to an ``int`` if ``Number.isSafeInteger()``
+returns ``true`` and otherwise we convert it to a ``float``. Conversely we
+convert ``float`` to ``number`` and we convert ``int`` to ``number`` unless it
+exceeds ``2**53`` in which case we convert it to a ``bigint``. We make a new
+subclass of ``int`` called ``JsBigInt`` to act as the conversion for
+``bigint``.``undefined`` is the default value for a missing argument so it
 corresponds to ``None``. We invent a new falsey singleton Python value
-``jsnull`` to act as the conversion of ``null``. We also make a new type
-``JsBigInt`` to act as the conversion for ``bigint``. All other types are
-proxied.
+``jsnull`` to act as the conversion of ``null``. All other types are proxied.
 
 In particular, even though ``tuples`` are immutable, they have no equivalent in
 JavaScript so we proxy them. They can be manually converted to an ``Array`` with