Clarify fetchart docs, and a couple 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 humbly propose these edits to the docs in an attempt to make
it more 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, not false, in order to prefer remote sources.
Merge pull request #259 from dsedivec/master

Clarify fetchart docs, and a couple corrections