Document a known problem with handling the process intended to receive
SIGIO in /dev/devctl.

Suggested by:	adrian
MFC after:	6 days
Make `-R', `-T' and `-E' options mutially non-exclusive. It is often
useful to see two or three types at the same time when inspecting the
dump.

MFC after:	1 month
Sponsored by:	Sippy Software, Inc.