Properly format documentation for `SignatureHelpRequest`s
Properly formats function documentation instead of returning it raw when responding to `SignatureHelpRequest`s.
I added a test in `crates/rust-analyzer/tests/slow-tests/main.rs` -- not sure if this is the best location given the relevant code is in `crates/rust-analyzer` or if it's possible to test in a less heavyweight manner.
Closes#14958