gh-150076: Document the public attributes of warnings.deprecated

The documentation described only the ``__deprecated__`` attribute set on
the decorated object, and did not mention that ``deprecated`` is a class
whose instances expose the constructor arguments (``message``, ``category``
and ``stacklevel``) as attributes of the same names -- a part of the API
that downstream projects rely on.

Document these attributes, and rename the first parameter in the signature
from ``msg`` to ``message`` to match the implementation and the attribute.

Author: fedonman

Doc/library/warnings.rst

@@ -566,7 +566,7 @@ Available Functions
    and calls to :func:`simplefilter`.
 
 
-.. decorator:: deprecated(msg, *, category=DeprecationWarning, stacklevel=1)
+.. decorator:: deprecated(message, *, category=DeprecationWarning, stacklevel=1)
 
    Decorator to indicate that a class, function or overload is deprecated.
 
@@ -612,6 +612,18 @@ Available Functions
    for the attribute to exist on the overload as returned by
    :func:`typing.get_overloads`.
 
+   ``deprecated`` is implemented as a class, so applying it (for example,
+   ``deprecated("Use B instead")``) returns an instance that acts as the
+   decorator.  The arguments passed to it are available as attributes of the
+   same names on that instance:
+
+   * ``message``: the deprecation message (a :class:`str`; a
+     :exc:`TypeError` is raised if it is not).
+   * ``category``: the warning category, or ``None`` if no runtime warning
+     should be emitted.
+   * ``stacklevel``: the number of stack frames to skip when emitting the
+     warning (an :class:`int`).
+
    .. versionadded:: 3.13
       See :pep:`702`.