docs: Document event-channel-based suspend protocol

docs: Document event-channel-based suspend protocol

Document the event channel protocol in xenstore-paths.markdown, in the
section for ~/device/suspend/event-channel.

Protocol reverse-engineered from commentary and commit messages of
  4539594d46f9  Add facility to get notification of domain suspend ...
  17636f47a474  Teach xc_save to use event-channel-based ...
and implementations in
  xc_save (current version)
  libxl (current version)
  linux-2.6.18-xen (mercurial 1241:2993033a77ca)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Shriram Rajagopalan <rshriram@cs.ubc.ca>