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.