Irritating Markdown syntax for headings conflicts with traditional comment frames in Lua

まだ誰も着手していません。

### How are you using the lua-language-server?

Visual Studio Code Extension (sumneko.lua)

### Which OS are you using?

Linux

### What is the issue affecting?

Annotations

### Expected Behaviour

"Hello World!" in the following annotation is NOT treated as a heading:
```
--------------------------------------------------------------------------------
--- Hello World!
--------------------------------------------------------------------------------
local function Main() end
```

### Actual Behaviour

"Hello World!" shows as a heading in the documentation pop-up window like so.

### Reproduction steps

Type the following code and hover your mouse over the function name.
```
--------------------------------------------------------------------------------
--- Hello World!
--------------------------------------------------------------------------------
local function Main() end
```

### Additional Notes

As someone who likes to frame documentation comments, I find this issue unbearable. The reason we should except Markdown's dash `----` syntax for headings in LuaCATS is because Lua uses dashes to delimit the comments themselves. And those who frame comments also use the same character (dash) for the rest of the frame. You wouldn't use another character for the frame, it would look ugly and not uniform. For example, in C, Markdown's heading syntax does not conflict with the comment delimiter (`//`). So you can frame your comments by repeating the comment delimiter itself for a uniform frame.

### Log File

Log files have no use in this issue.

レポートに示されている枠付きの Lua アノテーション内の関数にカーソルを合わせて、Visual Studio Code 拡張機能で問題を再現します。まず、アノテーションのドキュメントに使用されている LuaCATS の Markdown 見出し処理を見つけ、次に枠付きの「Hello World!」というテキストが見出しとしてレンダリングされなくなり、通常の Markdown 見出しは引き続き機能することを確認します。

索引モデルが issue の本文から書いたものです。