Feedback on updated SELinux docs

Overall, it looks really good. Kudos for a job well done.

I put in a couple of edits to try to improve a couple parts that seemed
a little hard to follow, but the main area for improvement I see is that
policy types are not discussed as a core concept. "Users and logins" mentions
targeted policy in the context of unconfined_u, but there's no preceding
section that could be linked in as a reference for more information.  
The "expert" section on policy store does mention the standard policy types, 
but it seems important enough topic that it deserves a mention in the
intro article (in particular, enough to guide user on choice between strict 
and targeted).