datetime.rst: improve combine() docs

The explanation on handling of `datetime` as the `date` arg was confusingly combined with unrelated text, and omitted proper formatting of arg names.

Author: belm0

Doc/library/datetime.rst

@@ -984,9 +984,10 @@ Other constructors, all class methods:
    is used.
 
    For any :class:`.datetime` object *d*,
-   ``d == datetime.combine(d.date(), d.time(), d.tzinfo)``. If date is a
-   :class:`.datetime` object, its time components and :attr:`.tzinfo` attributes
-   are ignored.
+   ``d == datetime.combine(d.date(), d.time(), d.tzinfo)``.
+   
+   If the *date* argument is a :class:`.datetime` object, its time components and
+   :attr:`.tzinfo` attributes are ignored.
 
    .. versionchanged:: 3.6
       Added the *tzinfo* argument.