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)