doc/radosgw: Improve rgw-cache.rst

doc/radosgw: Improve rgw-cache.rst

Try to improve the language by completely rewriting some sentences.
Attempt to format the document more like the rest of the docs.
Fix several errors in punctuation, capitalization, spaces etc.
Use blocks with bash prompts for CLI commands instead of hardcoded
prompts.
Fix section hierarchy and section title underline lengths.
Use admonition.

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