Update comments section of _beet
Update comments section of _beet

More accurate instructions on how to create a cached completion file (previous instructions where incorrect)
diego-plan9/beets,SusannaMaria/beets,beetbox/beets,diego-plan9/beets,madmouser1/beets,madmouser1/beets,shamangeorge/beets,diego-plan9/beets,pkess/beets,MyTunesFreeMusic/privacy-policy,sampsyo/beets,ibmibmibm/beets,xsteadfastx/beets,madmouser1/beets,artemutin/beets,MyTunesFreeMusic/privacy-policy,mosesfistos1/beetbox,shamangeorge/beets,ibmibmibm/beets,sampsyo/beets,Kraymer/beets,SusannaMaria/beets,mosesfistos1/beetbox,Kraymer/beets,MyTunesFreeMusic/privacy-policy,jackwilsdon/beets,mosesfistos1/beetbox,shamangeorge/beets,shamangeorge/beets,pkess/beets,Kraymer/beets,xsteadfastx/beets,lengtche/beets,madmouser1/beets,Kraymer/beets,lengtche/beets,sampsyo/beets,jackwilsdon/beets,ibmibmibm/beets,SusannaMaria/beets,mosesfistos1/beetbox,xsteadfastx/beets,xsteadfastx/beets,pkess/beets,lengtche/beets,jackwilsdon/beets,sampsyo/beets,lengtche/beets,ibmibmibm/beets,jackwilsdon/beets,artemutin/beets,pkess/beets,beetbox/beets,beetbox/beets,artemutin/beets,artemutin/beets,SusannaMaria/beets,MyTunesFreeMusic/privacy-policy,diego-plan9/beets,beetbox/beets