Mailbox: Update mailbox critical section documentation

Added information that mailbox critical section should be used to lock
data shared between secure and non-secure cores. And should not be used
to lock non-secure data that are not shared with secure core.

Signed-off-by: Roman Mazurak <roman.mazurak@infineon.com>
Change-Id: Ida178cc312c86011f79d24dd7f985533d66defd6