Defer all completion documentation creation until completionItem/resolve

[jakebailey]

https://microsoft.github.io/language-server-protocol/specification#completionItem_resolve

Instead of building documentation for everything and sending it for a completion request, this info should be omitted and filled in completionItem/resolve. This saves data for the initial request for docs that may never be viewed, and avoids running the (potentially slow) docstring converter for everything.

I believe there's a capability for this in the initialization that needs to be sent and verified.