added docstring for set_fields methods
added docstring for set_fields methods

pkess/beets,ibmibmibm/beets,SusannaMaria/beets,shamangeorge/beets,shamangeorge/beets,shamangeorge/beets,ibmibmibm/beets,jackwilsdon/beets,beetbox/beets,SusannaMaria/beets,ibmibmibm/beets,sampsyo/beets,beetbox/beets,jackwilsdon/beets,beetbox/beets,sampsyo/beets,pkess/beets,sampsyo/beets,SusannaMaria/beets,jackwilsdon/beets,SusannaMaria/beets,pkess/beets,sampsyo/beets,pkess/beets,shamangeorge/beets,beetbox/beets,ibmibmibm/beets,jackwilsdon/beets