doc/cephadm: small improvements to upgrade.rst

doc/cephadm: small improvements to upgrade.rst

Reorganize and split long text paragraphs.
Add a label for ref instead of using section title name.
Fix word capitalization and minor changes to words.

Use title case consistently in section titles.
Use double backticks consistently for literal strings.
Use confval role instead of literal inline for config keys in text.
Improve markup. Improve prompt usage.

Delete spaces at end of lines. Wrap long lines.

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