Clarify fetchart searching (and corrections)

Reading the fetchart docs it was not clear to me that it would use
_any_ image file found alongside your music files, even if the image
file did not have one of the five privileged names (cover, front, art,
album, folder).  I've attempted to rearrange this section of the docs
so it is clear that, by default, any local image file will be used.

I also corrected '"album," _for_ "folder"' to '"album," _or_
"folder"', and from reading the code I'm pretty sure that
remote_priority needs to be true in order to prefer remote sources,
not false.