userspace: document multiple mem domain init calls

k_mem_domain objects should not be initialized multiple times,
there's no support for memory domain life-cycles or freeing
memory that an arch_mem_domain_init() may have allocated.
Clearly document this.

The init function has to assume the provided domain is un-
initialized memory so it's not possible to robustly check
for this; a note is left in the arch_ definition to add an
assertion if feasible.

It's really unsafe to call an init function on any kernel
object more than once, but in this particular case if the
memory domain initialization resulted in the creation of
page tables or linked data structures really bad things can
happen. Currently no arch implements arch_mem_domain_init()
yet, but this is changing soon for x86.

One test case currently does this, it will be fixed in
a forthcoming patch.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>