docs: Explain what "role" and "status" means for output of microceph cluster list

*Please describe the question or issue you're facing with "MicroCeph CLI Commands - MicroCeph documentation".*

Role is not explained elsewhere - so unclear what is meaning of 'voter' and 'stand-by' and 'spare' (and how latter two are different)

Status - sometimes says unreachable when machine is up (as seen by ceph -s) and contactable (via ping) - so would be good to explain what is microceph definition of 'online' and 'unreachable'

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*Reported from: https://canonical-microceph.readthedocs-hosted.com/en/latest/reference/commands/*