Add comments why to avoid suggest converting doc comment to regular when missing a comma before doc in lists

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -686,6 +686,13 @@ impl<'a> Parser<'a> {
         }
 
         if let token::DocComment(kind, style, _) = self.token.kind {
+            // This is to avoid suggesting converting a doc comment to a regular comment
+            // when missing a comma before the doc comment in lists. (issue #142311)
+            // For example:
+            // `enum Foo{
+            //     A ///xxxxxxx
+            //     B,
+            // }`
             if !expected.contains(&TokenType::Comma) {
                 // We have something like `expr //!val` where the user likely meant `expr // !val`
                 let pos = self.token.span.lo() + BytePos(2);