doc/cephadm: small improvements to host-management.rst

doc/cephadm: small improvements to host-management.rst

Remove "*" around CLI command placeholders that was rendered in literal strings.
Leave only the usual "<placeholder>" string.

Use title case consistently in section titles.
Add links to more information.
Use double backticks consistently for literal strings.
Use confval role instead of literal inline for config keys in text.
Improve markup.

Fix word capitalization and minor changes to words.
Use Monitor, Manager, etc. instead of MON, MGR, etc.
Reword to "a command of the following form" where suitable.
Fix hyphenation.
Clarify three sentences.
SSH cert auth uses a signed public, not private, key.

Delete spaces at end of lines. Add full stops or colons.

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