add documentation for new lyrics flags
add documentation for new lyrics flags

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