doc/cephadm: Import existing ssh key docs.

This commit supersedes the commit below, which was suggested by
mohnewald:

1cd278b08a

Co-author: https://github.com/mohnewald

Signed-off-by: Zac Dover <zac.dover@gmail.com>

doc/cephadm/adoption.rst

@@ -81,6 +81,12 @@ Adoption process
 
      # ssh-copy-id -f -i ceph.pub root@<host>
 
+   .. note::
+     It is also possible to import an existing ssh key. See
+     :ref:`ssh errors <cephadm-ssh-errors>` in the troubleshooting
+     document for instructions describing how to import existing
+     ssh keys.
+
 #. Tell cephadm which hosts to manage::
 
      # ceph orch host add <hostname> [ip-address]

doc/cephadm/troubleshooting.rst

@@ -103,6 +103,7 @@ Cephadm writes small wrappers that run a containers. Refer to
 ``/var/lib/ceph/<cluster-fsid>/<service-name>/unit.run`` for the
 container execution command.
 
+.. _cephadm-ssh-errors:
 
 ssh errors
 ----------