MFC 302899: Add documentation for the sigevent structure.

MFC 302899: 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.

git-svn-id: svn://svn.freebsd.org/base/stable/9@303325 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f