doc/radosgw: improve DNS host name config in s3/commons.rst
Clarify that not only virtual-host-based access is configured this way.
Use Monitor config database instead of ceph.conf and add link to it.
Fix typo. Use privileged prompt for CLI example command.
List that comma is the delimitor used in rgw_dns_name config.
Use double backticks consistently for literal strings.
Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>