Clarify that "macro attributes" refers to proc macros

This also allows for the future possibility of `macro_rules`-based
attributes.

Author: joshtriplett

src/attributes.md

@@ -44,7 +44,7 @@ The following attributes are unsafe:
 Attributes can be classified into the following kinds:
 
 * [Built-in attributes]
-* [Macro attributes][attribute macros]
+* [Proc macro attributes][attribute macros]
 * [Derive macro helper attributes]
 * [Tool attributes](#tool-attributes)