Doc/memory-hotplug.txt: corrections and callback function prototype

Documentation/memory-hotplug.txt describes that a callback function can
be added to the notification chain by calling hotplug_memory_notifier().
The function prototype of the callback function is mssing. This missing
information is added by the patch.

The description of the arguments of the callback function is
reworked.

The constants for the event types are corrected.

The possible return values are explained.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>