doc/rados: fix sentences in health-checks (3 of x)

doc/rados: fix sentences in health-checks (3 of x)

Make sentences agree at the head of each section in
doc/rados/operations/health-checks.rst. The sentences were sometimes in
the imperative mood and sometimes in the declarative mood.

This commit edits the second third of
doc/rados/operations/health-checks.rst.

Note to (I hope soon) future Zac: There are a a couple of places near
the end of this file where the sentences are ungrammatical. Update these
in a separate PR (in isolation, so that the grammar and technical
accuracy of these sentences can be the primary focus of the reviewers).

Zac: cf. 000228

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