DOC: spoe: Clarify use of the event directive in spoe-message section

DOC: spoe: Clarify use of the event directive in spoe-message section

Only one event is possible for a spoe-message section. If defined several
time, only the last one is considered. The documentation is now explicit on
this point.

This patch is related to the the issue #1351.

(cherry picked from commit d322e948c979cff5bd419aa1b716186615651ba1)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 75a81ef8cacd5444615197980b1c774bd57dd8af)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit dadbbae0e7754791adca350f0dd4a64aabd8c78c)
Signed-off-by: Willy Tarreau <w@1wt.eu>