A little more documentation for `fields` (#1818)
A little more documentation for `fields` (#1818)

artemutin/beets,madmouser1/beets,mosesfistos1/beetbox,swt30/beets,xsteadfastx/beets,swt30/beets,mried/beets,Kraymer/beets,xsteadfastx/beets,ibmibmibm/beets,Kraymer/beets,MyTunesFreeMusic/privacy-policy,sampsyo/beets,madmouser1/beets,SusannaMaria/beets,pkess/beets,jackwilsdon/beets,jackwilsdon/beets,SusannaMaria/beets,jackwilsdon/beets,lengtche/beets,diego-plan9/beets,swt30/beets,diego-plan9/beets,MyTunesFreeMusic/privacy-policy,shamangeorge/beets,Freso/beets,lengtche/beets,mried/beets,artemutin/beets,artemutin/beets,ibmibmibm/beets,jcoady9/beets,pkess/beets,parapente/beets,jcoady9/beets,Kraymer/beets,beetbox/beets,artemutin/beets,xsteadfastx/beets,shamangeorge/beets,jackwilsdon/beets,pkess/beets,jcoady9/beets,diego-plan9/beets,SusannaMaria/beets,beetbox/beets,swt30/beets,xsteadfastx/beets,pkess/beets,MyTunesFreeMusic/privacy-policy,sampsyo/beets,madmouser1/beets,lengtche/beets,mosesfistos1/beetbox,parapente/beets,shamangeorge/beets,MyTunesFreeMusic/privacy-policy,parapente/beets,ibmibmibm/beets,Freso/beets,sampsyo/beets,lengtche/beets,diego-plan9/beets,mosesfistos1/beetbox,jcoady9/beets,mried/beets,parapente/beets,mried/beets,beetbox/beets,SusannaMaria/beets,beetbox/beets,sampsyo/beets,ibmibmibm/beets,Kraymer/beets,mosesfistos1/beetbox,madmouser1/beets,Freso/beets,shamangeorge/beets,Freso/beets