Change "forbidden lint" to "forbidden level"

Co-authored-by: Ryan Scheel <[email protected]>

Author: ehuss

src/attributes/diagnostics.md

@@ -40,7 +40,7 @@ pub mod m1 {
 ```
 
 Lint attributes can override the level specified from a previous attribute, as
-long as the level does not attempt to change a forbidden lint. Previous
+long as the level does not attempt to change a lint away from the forbidden level. Previous
 attributes are those from a higher level in the syntax tree, or from a
 previous attribute on the same entity as listed in left-to-right source order.