Add documentation for short_label in Helioviewer Event Interface

[dgarciabriseno]

This will support Helioviewer-Project/helioviewer.org#265 and dgarciabriseno/helioviewer-event-interface#11

This field should be added as part of the Helioviewer Event Interface field table in the appendix of the API Documentation.
The field represents the text that appears in the left sidebar.