Add prompt event documentation
Add prompt event documentation

* Add documentation for the before_choose_candidate event (add event to the
event list and add section with example usage).

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