rolling_update: clarify "serial" usage
rolling_update: clarify "serial" usage

Prior to this commit the serial variable was poorly documented. Now we
are making clear that this value should be left untouched as the rolling
update mechanism should happen serially.

Solves: bz-1396742
Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

fgal/ceph-ansible,WingkaiHo/ceph-ansible,font/ceph-ansible,WingkaiHo/ceph-ansible,albertomurillo/ceph-ansible,jtaleric/ceph-ansible,bengland2/ceph-ansible,albertomurillo/ceph-ansible,travmi/ceph-ansible,jsaintrocc/ceph-ansible,ceph/ceph-ansible,fgal/ceph-ansible,guits/ceph-ansible,font/ceph-ansible,WingkaiHo/ceph-ansible,ceph/ceph-ansible,travmi/ceph-ansible,jsaintrocc/ceph-ansible,jtaleric/ceph-ansible,albertomurillo/ceph-ansible,bengland2/ceph-ansible,guits/ceph-ansible