Describe so far undocumented functions and also the communication region
structure.
For the latter, we have to expand the generic COMM_REGION_GENERIC_HEADER
macro during a doxygen run. This is achieved by including the generic
header from within the arch-specific one, but only for doxygen
processing. This special treatment is required because doxygen processes
each file directly, even if it should have been processed indirectly
already (here asm/jailhouse_hypercall.h via jailhouse/hypercall.h).
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>