The commit c844bd87b3 removed the support
for passing a memory block (allocated in a memory pool) as message
data, but did not update the relevant documentation.
This commit removes any references to the memory block support, which
was removed in the v2.5.0 release, in the mailbox API documentation.
Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>
