Update comment.

Empty idents are also disambiguated. This was added in rust-lang#115367.

Author: nnethercote

compiler/rustc_resolve/src/lib.rs

@@ -534,8 +534,8 @@ struct BindingKey {
     /// identifier.
     ident: Ident,
     ns: Namespace,
-    /// 0 if ident is not `_`, otherwise a value that's unique to the specific
-    /// `_` in the expanded AST that introduced this binding.
+    /// 0 if ident is not `_` or ``, otherwise a value that's unique to the specific
+    /// `_`/`` in the expanded AST that introduced this binding.
     disambiguator: u32,
 }