doc/rados: Fix minor issues and improvements in health-checks.rst (2 of x)

doc/rados: Fix minor issues and improvements in health-checks.rst (2 of x)

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

Use RST **emphasis** instead of CAPS.

Use a privileged prompt for CLI command examples requiring privileges.

Capitalize Manager when talking about the service/dameon.

Use double backticks for literals instead of single backticks.

Remove spaces around slash in "and/or".

Add a paragraph referring OSD troubleshooting in OSD free space checks.

Modify flag lists to have a consistent style.

Reorder sentences to make more sense.

One small language improvement attempt.

Expand {a,b} glob used in text.

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