libvirt: Update virDomainSetMemory description

virDomainSetMemory is documented to change only runtime configuration of
running domain. However, that's not true of all hypervisors supported.
Seems as though when commit id '0f2e50be' added the current flag, the
function description should have been updated similar to when commit id
'c1795c52' updated the virDomainSetMaxMemory description. Especially since
commit id '80427f1d' updated the virsh 'setmem' description to indicate
"behavior is different depending on hypervisor."

This patch will update the description to match current functionality.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>