fetchart: better document 'sources' option
fetchart: better document 'sources' option

PierreRust/beets,mosesfistos1/beetbox,asteven/beets,lengtche/beets,dfc/beets,marcuskrahl/beets,tima/beets,parapente/beets,ibmibmibm/beets,marcuskrahl/beets,multikatt/beets,kelvinhammond/beets,madmouser1/beets,kareemallen/beets,jackwilsdon/beets,sampsyo/beets,jackwilsdon/beets,mried/beets,ruippeixotog/beets,LordSputnik/beets,swt30/beets,ibmibmibm/beets,jmwatte/beets,pkess/beets,mosesfistos1/beetbox,Freso/beets,ttsda/beets,LordSputnik/beets,lightwang1/beets,lengtche/beets,Wen777/beets,YetAnotherNerd/beets,ruippeixotog/beets,tima/beets,beetbox/beets,drm00/beets,lightwang1/beets,Andypsamp/CODfinalJUNIT,shanemikel/beets,kelvinhammond/beets,jmwatte/beets,kareemallen/beets,untitaker/beets,beetbox/beets,dfc/beets,andremiller/beets,MyTunesFreeMusic/privacy-policy,mathstuf/beets,asteven/beets,YetAnotherNerd/beets,Freso/beets,artemutin/beets,Andypsamp/CODfinalJUNIT,PierreRust/beets,randybias/beets,LordSputnik/beets,imsparsh/beets,Dishwishy/beets,arabenjamin/beets,beetbox/beets,jmwatte/beets,swt30/beets,multikatt/beets,diego-plan9/beets,mosesfistos1/beetbox,Andypsamp/CODfinalJUNIT,SusannaMaria/beets,mried/beets,randybias/beets,marcuskrahl/beets,Kraymer/beets,MyTunesFreeMusic/privacy-policy,moodboom/beets,shanemikel/beets,artemutin/beets,shanemikel/beets,madmouser1/beets,kareemallen/beets,YetAnotherNerd/beets,Dishwishy/beets,andremiller/beets,ruippeixotog/beets,ttsda/beets,MyTunesFreeMusic/privacy-policy,shanemikel/beets,kelvinhammond/beets,artemutin/beets,xsteadfastx/beets,Andypsamp/CODjunit,moodboom/beets,andremiller/beets,shamangeorge/beets,parapente/beets,ibmibmibm/beets,mried/beets,artemutin/beets,shamangeorge/beets,Andypsamp/CODjunit,xsteadfastx/beets,SusannaMaria/beets,ruippeixotog/beets,swt30/beets,Freso/beets,untitaker/beets,asteven/beets,madmouser1/beets,sadatay/beets,lengtche/beets,mosesfistos1/beetbox,drm00/beets,jcoady9/beets,parapente/beets,untitaker/beets,Andypsamp/CODjunit,jackwilsdon/beets,lengtche/beets,LordSputnik/beets,shamangeorge/beets,ttsda/beets,moodboom/beets,madmouser1/beets,kareemallen/beets,Andypsamp/CODfinalJUNIT,jmwatte/beets,PierreRust/beets,sadatay/beets,Freso/beets,jackwilsdon/beets,imsparsh/beets,jcoady9/beets,xsteadfastx/beets,Wen777/beets,parapente/beets,MyTunesFreeMusic/privacy-policy,shamangeorge/beets,PierreRust/beets,arabenjamin/beets,marcuskrahl/beets,randybias/beets,m-urban/beets,SusannaMaria/beets,mathstuf/beets,pkess/beets,ibmibmibm/beets,Kraymer/beets,m-urban/beets,sampsyo/beets,lightwang1/beets,drm00/beets,dfc/beets,gabrielaraujof/beets,asteven/beets,Wen777/beets,multikatt/beets,moodboom/beets,gabrielaraujof/beets,mried/beets,sadatay/beets,multikatt/beets,drm00/beets,jcoady9/beets,arabenjamin/beets,Andypsamp/CODfinalJUNIT,imsparsh/beets,Dishwishy/beets,diego-plan9/beets,sadatay/beets,sampsyo/beets,lightwang1/beets,SusannaMaria/beets,gabrielaraujof/beets,swt30/beets,Andypsamp/CODjunit,Andypsamp/CODjunit,xsteadfastx/beets,sampsyo/beets,Kraymer/beets,imsparsh/beets,kelvinhammond/beets,ttsda/beets,pkess/beets,Dishwishy/beets,untitaker/beets,beetbox/beets,dfc/beets,pkess/beets,gabrielaraujof/beets,YetAnotherNerd/beets,jcoady9/beets,tima/beets,mathstuf/beets,diego-plan9/beets,randybias/beets,tima/beets,m-urban/beets,arabenjamin/beets,m-urban/beets,mathstuf/beets,diego-plan9/beets,Kraymer/beets