doc/rados/operations: document the kernel-client min-compat holdout

doc/rados/operations: document the kernel-client min-compat holdout

A kernel CephFS or RBD mount can keep ``set-require-min-compat-client
reef`` from succeeding: the kernel client does not advertise the
pg-upmap-primary feature, so ``ceph features`` reports it as a luminous
client even when the daemons and every userspace client are newer.
ceph-fuse, libcephfs, and librbd advertise the full feature set and are
not affected.

Document this, and add a section on finding the holdout with
``ceph features`` and switching it to a userspace client.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>