doc/mgr: Small improvements in rgw.rst

doc/mgr: Small improvements in rgw.rst

Use double backticks consistently for module name, CLI commands and
parameters etc.

Use title case in section titles and underline them only until end of
title text.

Use "command line" instead of "cmd line" in text.

Capitalize RGW consistently.

Delete one paragraph that included a strange out of place text fragment.

Call it "Manager" consistently instead of "MGR" in text one time.

Improve language in CA cert upgrade.

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