Merge #4903

4903: Add highlighting support for doc comments r=matklad a=Nashenas88

The language server protocol includes a semantic modifier for documentation. This change exports that modifier for doc comments so users can choose to highlight them differently compared to regular comments.

Example:
<img width="375" alt="Screen Shot 2020-06-16 at 10 34 14 AM" src="https://user-images.githubusercontent.com/1673130/84788271-f6599580-afbc-11ea-96e5-7a0215da620b.png">

CC @woody77 

Co-authored-by: Paul Daniel Faria <Nashenas88@users.noreply.github.com>

crates/ra_ide/src/syntax_highlighting.rs

@@ -489,7 +489,14 @@ fn highlight_element(
         }
 
         // Simple token-based highlighting
-        COMMENT => HighlightTag::Comment.into(),
+        COMMENT => {
+            let comment = element.into_token().and_then(ast::Comment::cast)?;
+            let h = HighlightTag::Comment;
+            match comment.kind().doc {
+                Some(_) => h | HighlightModifier::Documentation,
+                None => h.into(),
+            }
+        }
         STRING | RAW_STRING | RAW_BYTE_STRING | BYTE_STRING => HighlightTag::StringLiteral.into(),
         ATTR => HighlightTag::Attribute.into(),
         INT_NUMBER | FLOAT_NUMBER => HighlightTag::NumericLiteral.into(),