Clarify details around fence and progbuf. (#601)

* Clarify details around fence and progbuf.

Addresses #533.

* Fix typo.

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

* "output," not "I/O"

* Remove all mentions of fence.

I was going to move this into the appendix, but I realize I don't even
understand why it needs to be mentioned there.

* Mention fence in appendix

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

* Rephrase.

I think this says the same thing, but more concisely.

* Give example of side effect we need the fence for.

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>