man/openrc.8: update checkpath documentation

X-GENTOO-BUG: 500606
X-GENTOO-BUG-URL: https://bugs.gentoo.org/show_bug.cgi?id=500606
man/openrc-run.8: more updates and clarifications for checkpath

X-Gentoo-Bug: 500606
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=500606