Add documentation for the sigevent structure.

• Add a sigevent(3) manpage to give a general overview of the sigevent structure and the available notification mechanisms.
• Document that AIO requests contain a nested sigevent structure that can be used to request completion notification.
• Expand the sigevent details in other manuals to note details such as the extra values stored in a queued signal's information or in a posted kevent.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D7122