xl: update the manpage about "cpus=" and NUMA node-affinity

xl: update the manpage about "cpus=" and NUMA node-affinity

Since d06b1bf169a01a9c7b0947d7825e58cb455a0ba5 ('libxl: automatic placement
deals with node-affinity') it is no longer true that, if no "cpus=" option
is specified, xl picks up some pCPUs by default and pin the domain there.

In fact, it is the NUMA node-affinity that is affected by automatic
placement, not vCPU to pCPU pinning.

Update the xl config file documenation accordingly, as it seems to have
been forgotten at that time.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>