doc/rados/operations: add kernel client procedure to read balancer documentation

doc/rados/operations: add kernel client procedure to read balancer documentation

As of now, the kernel client does not support `pg-upmap-primary`. I have
added some troubleshooting steps to help users who are unable to
mount images and filesystems with the kernel client while using `pg-upmap-primary`.

Once the feature is supported by the kernel client, users will be able
to perform mounts along with `pg-upmap-primary`.

Fixes: https://tracker.ceph.com/issues/72897
Signed-off-by: Laura Flores <lflores@ibm.com>
(cherry picked from commit 546d523147873c8fc6cf4208b4fa71eb2703e9c3)