Add documentation on EVENT_READ/EVENT_WRITE constants

Add documentation on EVENT_READ/EVENT_WRITE constants

Changes from v4:
- get rid of the overly complex EVENT_SHIFT() macro

Changes from v3:
- re-introduce READ/WRITE_SHIFT because they are different from
EVENT_READ/WRITE
- define also EVENT_READ/WRITE using READ/WRITE_SHIFT

Changes from v2:
- moved event definitions to event.h
- removed READ/WRITE_SHIFT and use EVENT_READ/WRITE
- removed ifdefs around *_SHIFTS definitions in event.h

Changes from v1:
- fixed typ0s
- extended comment
- moved *_SHIFT definition to openvpn.h
- made READ/WRITE events dependant on _SHIFT definition with a macro

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20210427191314.21317-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22247.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>