Add javadoc descriptions to generated types

Documentation can be combined from multiple sources. Probably the best option atm is to gather type documentation from [apiDescriptions.json](https://github.com/Microsoft/TSJS-lib-generator/blob/master/inputfiles/mdn/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.