Include documentation from a variable's type when you hover the variable

Issue Type: Feature request

I had hoped to see the description for the type when hovering over the value via intellisense, etc.

When I hover over context.slot I can see "The slot the disk is in." yet when I try hovering over const slot I see just the type. Is there anything to enable this? Even setting slot to Slot manually it still only shows the type.

 /** The slot the disk is in. */
type Slot = string;

interface Context {
    /** The slot the disk is in. */
    slot?: Slot
}

const func = (context: Context): Context => {
    const x = context.slot;
    const slot: Slot = context.slot;

    return {
        slot: x || slot
    };
};

func({
    slot: '1'
}).slot;

VS Code version: Code 1.43.2 (0ba0ca52957102ca3527cf479571617f0de6ed50, 2020-03-24T07:34:57.037Z)
OS version: Darwin x64 19.2.0

まず、issue の TypeScript の例で hover の動作を再現し、VS Code で使用されている TypeScript language-service の hover パスを調査します。型がエイリアスを参照している変数について、ドキュメントがどのように選択されるかを確認し、そのうえで、既存のプロパティの hover 動作を変更せずに、hover に型のドキュメントが含まれることを検証します。