Add JSDocs style commenting to exported "types"

[jplante]

Ideally it would be great if the TypeDocs JSON output could provide "type" spefici information about exported/declared types.

For example:

export type VectorAlias = 
  | Vector 
  | { direction: number, magnitude: number } 
  | [ direction: number, magnitude: number ]
;

Could become:

export type VectorAlias = 
 /**
  * description blah blah blah
  */   
  | Vector 

 /**
  * description blah blah blah
  */   
  | { direction: number, magnitude: number } 

 /**
  * description blah blah blah
  */   
  | [ direction: number, magnitude: number ]
;

[Gerrit0]

This is... tricky. Where does this end?

type Something =
  /** 1 */
  | A
  | { /** 2 */ b: 2 }
  | { c: /** 3 */ 3 }
  | { d: [/** 4 */ 4 | 5] }

[Gerrit0]

I'm going to go ahead and mark this as wontfix - I do plan on adding support for "union enums", but that's a very specific case. I haven't been able to figure out a nice way to generically handle comments everywhere microsoft/tsdoc#164