doc/rados: Update mClock doc on steps to override OSD IOPS capacity config

doc/rados: Update mClock doc on steps to override OSD IOPS capacity config

Describe the steps involved to
 - Specify a global value for osd_mclock_max_capacity_iops_{ssd,hdd}, and
 - Override existing individually scoped values for OSDs determined during
   start-up for osd_mclock_max_capacity_iops_{ssd,hdd}.

The above is to help with the following:
 - Steps to override existing setting with a global value.
 - reduce the number of entries in the mon store and instead use a single
   global specification for all OSDs in the cluster in case the underlying
   hardware is the same for all OSDs.

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>
Fixes: https://tracker.ceph.com/issues/70774