ENH: Fleshed out description of event_desc keys
ENH: Fleshed out description of event_desc keys

- Improved the description of the event descriptor keys
  dictionary. Done with @tacaswell

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