Document the 3 fields of `ReasonElem` in its Haddocks
Document the 3 fields of `ReasonElem` in its Haddocks

From the reason trail design doc
(http://docs.ganeti.org/ganeti/master/html/design-reason-trail.html).

Signed-off-by: Niklas Hambuechen <98215d3a72cb13cc9c9956528e50119ca7d1fb8b@google.com>
Signed-off-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>
Reviewed-by: Klaus Aehlig <4ce51de36d9068cd7db2eec1151e0f3f95419008@google.com>

andir/ganeti,ganeti/ganeti,yiannist/ganeti,andir/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,bitemyapp/ganeti,dimara/ganeti,mbakke/ganeti,dimara/ganeti,grnet/snf-ganeti,ganeti/ganeti,grnet/snf-ganeti,yiannist/ganeti,onponomarev/ganeti,mbakke/ganeti,apyrgio/ganeti,leshchevds/ganeti,andir/ganeti,ganeti/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,yiannist/ganeti,bitemyapp/ganeti,mbakke/ganeti,apyrgio/ganeti,leshchevds/ganeti