doc/cephadm: small improvements continued

doc/cephadm: small improvements continued

Fix invalid section title and add a label for ref.
Link to the new label instead of telling which section to read.
Remove a sentence no longer relevant with config db.

Remove stray spaces, add newlines per style, fix capitalization.
Use privileged prompt when necessary.

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