doc/rados: explain replaceable parts of command

doc/rados: explain replaceable parts of command

Add an explanation that directs the reader to replace the "X" part of
the command "ceph tell mon.X mon_status" with the value specific to the
reader's Ceph cluster (which is (probably) not "X").

In the future, such replaceable strings in commands may be bounded by
angle brackets ("<" and ">").

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/

Signed-off-by: Zac Dover <zac.dover@proton.me>