doc/radosgw: Promptify cmds and improve formatting in placement.rst

doc/radosgw: Promptify cmds and improve formatting in placement.rst

Use preformatted blocks with a privileged bash prompt instead of
hardcoding prompts in the beginning of each line for CLI commands.

Indent continuation lines of multi-line CLI example commands the same
way they are indented elsewhere.

Use inline code formatting consistently, add double-backticks for
inside text references to CLI commands, configuration data, etc.

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