Improve http_host, filename in docs and example/http-s3.fio

Improve http_host, filename in docs and example/http-s3.fio

In fio.1 and HOWTO.rst:
  * http_host: add details on vHost- vs Path-style hostname
    differences
  * filename: add details on bucket prefix for Path-style hostname
In examples/http-s3.fio:
  * add comments at the top to clarify vHost vs Path S3 and change
    the hostname in existing example to disambiguate two scenarios

Signed-off-by: Sean Lee <sean.lee@netapp.com>