virsh: Fix docs for "virsh setmaxmem"

The docs assumed the command works always for QEMU and other
hypervisors. As this is done using the balloon mechainism live increase
of the maximum memory limit isn't supported. Fix the docs to mention
this limitation.

tools/virsh.pod

@@ -1477,7 +1477,8 @@ Both I<--live> and I<--config> flags may be given, but I<--current> is
 exclusive. If no flag is specified, behavior is different depending
 on hypervisor.
 
-This command works for at least the Xen, QEMU/KVM and vSphere/ESX hypervisors.
+Some hypervisors such as QEMU/KVM don't support live changes (especially
+increasing) of the maximum memory limit.
 
 I<size> is a scaled integer (see B<NOTES> above); it defaults to kibibytes
 (blocks of 1024 bytes) unless you provide a suffix (and the older option