The explicit event dispatch sequence currently depicts handling done in
Secure EL1, although further error handling is typically done inside a
Secure Partition. Clarify the sequence diagram to that effect.

Change-Id: I53deedc6d5ee0706626890067950c2c541a62c78
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>