coc should unescape the text of the markdown hover card

[jsholmes]

Result from CocInfo

## versions

vim version: NVIM v0.5.0-dev
node version: v14.1.0
coc.nvim version: 0.0.78-be094451df
term: xterm-256color
platform: linux

## Output channel: clangd log

I[11:29:31.424] clangd version 9.0.1-8+hrt1hrt9.1
I[11:29:31.425] Working directory: /usr/scratch/john/hrt/dev
I[11:29:31.425] argv[0]: /usr/bin/clangd-9
I[11:29:31.425] Starting LSP over stdin/stdout
...

Describe the bug

coc requests the markdown version of the hover card, but then doesn't unescape the text per markdown rules.

This leaves the hover card pretty unreadable.

See clangd/coc-clangd#48 (comment)

Screenshots (optional)

Screenshot from cited issue for coc-clangd:

[fannheyward]

clangd/coc-clangd#48 (comment) looks like this is a server error, close and track on clangd/coc-clangd#48