DOC: api/event_hdl: small updates, fix an example and add some precisions

Fix an example suggesting that using EVENT_HDL_SUB_TYPE(x, y) with y being
0 was valid. Then add some notes to explain how to use
EVENT_HDL_SUB_FAMILY() and EVENT_HDL_SUB_TYPE() with valid values.

Also mention that the feature is available starting from 2.8 and not 2.7.
Finally, perform some purely cosmetic updates.

This could be backported in 2.8.

(cherry picked from commit 13e0972aeac275137b429163def950af88fecd46)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 75a9d54c6718e3accad91c624d8781a29f1e3d09)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 371102afeb961b516df133d343a39248a08e0275)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
1 file changed