LSP: Return appropriate markdown content

[kjeremy]

If HoverClientCapabilities.contentFormat or CompletionClientCapabilities.completionItem.documentationFormat or SignatureHelpClientCapabilities.signatureInformation.documentationFormat are absent then we need to return:

Request	Return
Hover	MarkedString
Completion	string
SignatureHelp	string

See: https://microsoft.github.io/language-server-protocol/specifications/specification-3-16/#textDocument_hover

[kjeremy]

This is part of #144

[kjeremy]

User report in the wild: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Fwg-rls-2.2E0/topic/MarkupKind/near/201324222