Use TextDocumentClientCapabilities for documentation kind decision

[jakebailey]

We have a custom option to set the preferred documentation kind in initializationOptions, but the LS spec provides a way to do this in TextDocumentClientCapabilities:

completionItem?: {
	// ...

	/**
	 * Client supports the follow content formats for the documentation
	 * property. The order describes the preferred format of the client.
	 */
	documentationFormat?: MarkupKind[];

	// ...
}

Additionally, MarkupKind is duplicated, and the logic for choosing which to use seems to be reimplemented in a few different places that produce MarkupContent.

[MikhailArkhipov]

Yeah, we pass it in init b/c that's the way VS has been doing it. Markdown == VSC, plain text == VS. This can be part of the PTVS update item #32 .

[jakebailey]

Yup, I wasn't sure how this would fit with VS, considering that it doesn't actually communicate over RPC. We seem to do a lot of stuff with settings in the language server init/settings change RPC handlers that I wish were placed elsewhere to be more consistent/testable (not to mention the duplication of many structs between the analyzer and LS).

[jakebailey]

Closing in favor of #608.