doc: Improve man page for bench

Add information about seq and rand for bench. Also mention
--no-cleanup option.

Signed-off-by: Kevin Dalley <kevin@kelphead.org>
doc: Improve man page for rados bench

Add information about seq and rand for bench. Also mention
--no-cleanup option.

Signed-off-by: Kevin Dalley <kevin@kelphead.org>
Merge pull request #1885 from nereocystis/rados-bench-man

doc: Improve man page for bench