Document the -r option in arcstat(1M) per illumos bug 12690. Note that there is also a single line removed (both because it makes the source easier to read and because it showed up in the final product displayed in man) which I fixed while I was already in this file. If really necessary I can split that into a separate bug/commit/review set, but it just really didn't seem worth it.

I previewed this in man (albeit on a Debian system) and ran it through mandoc -Tlint. I didn't want to build all of illumos-gate just to get git-pbchk so I asked on IRC if someone would be willing to run it for me (I'm waiting to hear back).