rfc: cfg_attr-multiple-attrs: Emit unused_attributes warning specifically

Author: Havvy

text/0000-cfg_attr-multiple-attrs.md

@@ -107,7 +107,7 @@ is valid. This example would be equivalent to
 ## Warning When Zero Attributes
 
 This RFC allows `#[cfg_attr(predicate)]`. This is so that macros can generate
-it. Having it in the source text emits a warning.
+it. Having it in the source text emits an `unused_attributes` warning.
 
 ## Attribute Syntax Opportunity Cost