Add javadoc descriptions to generated types

[realityforge]

Documentation can be combined from multiple sources. Probably the best option atm is to gather type documentation from apiDescriptions.json as most of the hard work is done and it seems to be kept uptodate.

The per function and property documentation could be grabbed from either:

• the closure externs (easiest option) or
• the WebIDLs. This is a lot harder and may not be worth the effort unless other benefits would be gathered from running a WebIDL reader etc. See https://github.com/Microsoft/TSJS-lib-generator/tree/master/inputfiles/idl for a copy of WebIDL that would likely serve as input.

[jDramaix]

I think the best is:

• update the closure externs documentation ( with apiDescriptions.json or whatever) and commit the change in their repo
• Modifying the change for generating javadoc from jsdoc. I prototyped that a while ago, I can resurrect my internal cl and submit it.