Improve some option descriptions

Deals with most of the remaining concerns from #1140.
Wording may be imperfect, so feel free to critique it.

I wasn't sure about "Prefer RES Albums" - when I turned it off, no expandos showed up at all for imgur albums (and it's been so long since I've used reddit without RES that I don't remember if there were any).

I'm also not sure if pinHeader needs to be clarified or if it already has been (it doesn't seem too unintuitive right now).