Fix/improve typings of internal _eventContext properties, add some docs for event context functions

[ericyhwang]

• [internal] Fix typing of Model#_eventContext to be a string, not a number
  • Derby component instances pass component id string to eventContext(id) here: https://github.com/derbyjs/derby/blob/9968348205560323c592aa3f81e38ff8ffdc62e8/src/components.ts#L72-L74
  • Component id generation's implementation is here: https://github.com/derbyjs/derby/blob/9968348205560323c592aa3f81e38ff8ffdc62e8/src/templates/contexts.ts#L88-L95
• Add JSDoc to event context methods/properties