doc/radosgw: Improve language and formatting in config-ref.rst

doc/radosgw: Improve language and formatting in config-ref.rst

The "Topic persistency settings" section includes convoluted
and incomplete sentences so attempt to improve the
language. The section lacks context so add a link to the
relevant documentation about Notifications.

The "Garbage Collection settings" note includes a sentence
that looks incomplete so try to improve the language.

Don't capitalize "Ops" in "RGW Ops" especially when the
same sentence talks about "op queue" with lower case "o".

Also improve formatting: indentation of a note and wrap
several very long lines.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>
(cherry picked from commit 9dd86773b39582322a5577d7cc1521486c1bdaa9)