doc/rados: improve leader/peon monitor explanation

doc/rados: improve leader/peon monitor explanation

Add an explanation of leader-peon conditions that obtain when the
cluster is in the "HEALTH_OK" state. Previously, the text discussed
these two monitor states only in the context of a health detail entry.

This improvement to the documentation was suggested on the [ceph-users]
email list by Joel Davidow. This email, an absolute model of user
engagement with an upstream project, can be reviewed here: https://lists.ceph.io/hyperkitty/list/ceph-users@ceph.io/message/KF67F5TXFSSTPXV7EKL6JKLA5KZQDLDQ/

I will list Joel Davidow here as the co-author for the sake of more
expediently getting this change into the documentation, but though he is
listed as the co-author, he is the true author.

Co-authored-by: Joel Davidow <jdavidow@nso.edu>
Signed-off-by: Zac Dover <zac.dover@proton.me>