fix(render): not render two separator for doc window

[tim3nd]

Some LSP servers like lua_ls would return --- for function doc, so the manually added separator would make two separator in doc window.

This PR adds some specialized logic to handle this, tested for lua_ls and rust analyzer (lsp server won't return separator), work fine for now.

Before:

• lua_ls (would return --- for function doc)
  

• rust analyzer
  

After:

• lua_ls (would return --- for function doc)
  

• rust analyzer (works like before)
  

[tim3nd]

Update:
Found some outdated highlight groups in README, already updated for now. Maybe we can merge it together if don't mind :)

[Saghen]

Thanks again!