doc/rgw: use 'confval' directive to render sts config options

doc/rgw: use 'confval' directive to render sts config options

the 'confval' directive reads the config options from
common/options/rgw.yaml and renders them nicely. this keeps
everything consistent between the options and their docs

improve the config option descriptions:

* add existing note about rgw_sts_key length/format
* add example openssl command to generate a conforming sts key
* add notes about sharing sts key between gateways/zones

format the last remaining 'Note' with the 'note' directive

Signed-off-by: Casey Bodley <cbodley@redhat.com>