Split outer attribute definition to a dedicated rule

Author: ehuss

src/attributes.md

@@ -24,7 +24,10 @@ on Attributes in [ECMA-335], with the syntax coming from [ECMA-334] \(C#).
 
 r[attributes.inner]
 _Inner attributes_, written with a bang (`!`) after the hash (`#`), apply to the
-item that the attribute is declared within. _Outer attributes_, written without
+item that the attribute is declared within.
+
+r[attributes.outer]
+_Outer attributes_, written without
 the bang after the hash, apply to the thing that follows the attribute.
 
 r[attributes.input]