Add field description

Add a short description explaining `field_event_date` exists and what
it's used for.

config/sync/field.field.node.talk.field_event_date.yml

@@ -12,7 +12,7 @@ field_name: field_event_date
 entity_type: node
 bundle: talk
 label: 'Event date'
-description: ''
+description: 'An automatically updated field that stores the date of the last event for this talk, which makes it easier to sort the listing page.'
 required: false
 translatable: false
 default_value: {  }