docs: Document memory bandwidth allocation limits more clearly

docs: Document memory bandwidth allocation limits more clearly

The meaning of the values as well as their maximums are hard to predict
and accounting for all the possibilities (which by the way might change
during daemon's execution) is borderline hallucinatory.  There is
already a way we represent them, which is the same as the Linux kernel.
We do not interpret them at all, just blindly use them.  In order to
make this more apparent for the users change the documentation for the
<memorytune/> (not <memtune/>) element more boldly.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>