DOC : update event descriptor schema
DOC : update event descriptor schema

 - added missing begin_run_event link
 - made time mandatory
 - improved descriptions a bit

NSLS-II/docs,danielballan/docs,waynelewis/nsls-ii-docs,NSLS-II/docs,NSLS-II-SRX/docs,tacaswell/docs,danielballan/docs,waynelewis/nsls-ii-docs,NSLS-II-SRX/docs