Fixed markdown in datatips

[autumnull]

The datatips shown on hover were just showing malformed plaintext documentation rather than rendered markdown. This should fix it.

[aminya]

Could you post a screenshot?

[autumnull]

Before code changes:

After code changes:

This issue appeared a couple of months back and is rooted in the fact that rust-analyzer requires you to let it know that you support markdown documentation or else it will give you plaintext documentation. I have tested this with a few of the latest versions of rust-analyzer from different sources but this particular screenshot is with

$ rust-analyzer --version
rust-analyzer 2022-03-07