Docs: Clarify IRQ/FIQ handler API in Interrupt Design Guide

Docs: Clarify IRQ/FIQ handler API in Interrupt Design Guide

The API description currently states that interrupt handlers
(interrupt_type_handler_t pointers) SHOULD return a pointer
to the target cpu_context.

This patch rewords the description of the interrupts handlers to state
that it MUST return a pointer to the target security context. It also
warns about potential portability issue. Specifically, this pointer is
not used on AArch64 and calls to context library APIs must be used to
ask BL31 to return to a specific context. However, this could change in
the future and interrupt handlers must always return the pointer.

Change-Id: I3f82a046de4d7a5b51a8cbebe7eb2a00dbbdb4f0
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>