Extend the listing of how the formats work

Author: DavidCEllis

Doc/library/annotationlib.rst

@@ -340,14 +340,29 @@ Functions
 
    * VALUE: :attr:`!object.__annotations__` is tried first; if that does not exist,
      the :attr:`!object.__annotate__` function is called if it exists.
+
    * FORWARDREF: If :attr:`!object.__annotations__` exists and can be evaluated successfully,
      it is used; otherwise, the :attr:`!object.__annotate__` function is called. If it
      does not exist either, :attr:`!object.__annotations__` is tried again and any error
      from accessing it is re-raised.
+
+     * When calling :attr:`!object.__annotate__` it is first called with :attr:`~Format.FORWARDREF`,
+       if this is not implemented it will then check if :attr:`~Format.VALUE_WITH_FAKE_GLOBALS`
+       is supported and use that in the fake globals environment.
+       If neither of these formats are supported it will fall back to using :attr:`~Format.VALUE`.
+       If :attr:`~Format.VALUE` fails, the error from this call will be raised.
+
    * STRING: If :attr:`!object.__annotate__` exists, it is called first;
      otherwise, :attr:`!object.__annotations__` is used and stringified
      using :func:`annotations_to_string`.
 
+     * When calling :attr:`!object.__annotate__` it is first called with :attr:`~Format.STRING`,
+       if this is not implemented it will then check if :attr:`~Format.VALUE_WITH_FAKE_GLOBALS`
+       is supported and use that in the fake globals environment.
+       If neither of these formats are supported it will fall back to using :attr:`~Format.VALUE`
+       with the result converted using :func:`annotations_to_string`.
+       If :attr:`~Format.VALUE` fails, the error from this call will be raised.
+
    Returns a dict. :func:`!get_annotations` returns a new dict every time
    it's called; calling it twice on the same object will return two
    different but equivalent dicts.