Add "only".

Co-Authored-By: ehuss <[email protected]>

Author: Havvy

src/attributes.md

@@ -582,7 +582,7 @@ struct S {
 pub fn f() {}
 ```
 
-> Note: `rustc` currently recognizes the tools "clippy" and "rustfmt".
+> Note: `rustc` currently only recognizes the tools "clippy" and "rustfmt".
 
 [_DelimTokenTree_]: macros.html
 [_LiteralExpression_]: expressions/literal-expr.html