doc/rados: Fix minor issues and improvements in health-checks.rst

doc/rados: Fix minor issues and improvements in health-checks.rst

Use title case consistently in section titles.

Wrap long lines over 80 characters.

Use "Ceph Manager" instead of "ceph-mgr".
Use "Ceph Monitor" or "Monitor" consistently instead of "monitor".

Use :confval: instead of simple inline literal text for config values.

Linkify, adding a label in the destination if necessary:
- Muting health checks
- Mon election strategy

Fix a typo.

Use RST *italic* instead of _rendered underscores_.

Use a privileged prompt for CLI command examples requiring privileges.

Use path including fsid to mon data consistently.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>