iostat.8: Better wording for some use cases
The iostat(8) manual page should be more specific when using
options. Also extented the BUGS section.
PR: 153012
Reviewed by: bcr
Differential Revision: https://reviews.freebsd.org/D42633
(cherry picked from commit 502e8f44fcc18bfc6dadca56801d66cbce4c07ec)