Fix client capabilities in window/progress proposed doc

[Xanewok]

Fixes wrong ClientCapabilities extension from #261.

@dbaeumer by the way, what is the workflow for proposed features? Do they become a part of a regular protocol version but noted that they're unstable and only proposed at the moment?

[dbaeumer]

@Xanewok thanks. I already started fixing this on the correspoding progress branch I have.

While implementing cancel support I separated the notification into start, report, done and cancel. Mainly because it was very hard to descibe in which cases which property combinations were valid. Was a lot easier having separate literals per request. I hope this is OK with you.

For going forward with the proposal: I added a comment here microsoft/language-server-protocol#70 (comment) to get additional feedback on the proposal. If there is no strong objection I usually merge the proposal into the final version.

I will also publish new next version of the node libraries containing the implementation.