Improve long explanation for E0546

Author: jesusprubio

compiler/rustc_error_codes/src/error_codes/E0546.md

@@ -1,4 +1,4 @@
-A feature name is missing.
+The `feature` value is missing in a stability attribute.
 
 Erroneous code example:
 
@@ -13,15 +13,15 @@ fn unstable_fn() {}
 fn stable_fn() {}
 ```
 
-To fix the issue you need to provide a feature name.
+To fix the issue you need to provide the `feature` field.
 
 ```
 #![feature(staged_api)]
 #![stable(since = "1.0.0", feature = "test")]
 
-#[unstable(feature = "unstable_fn", issue = "none")] // ok!
+#[unstable(feature = "unstable_fn", issue = "none")]
 fn unstable_fn() {}
 
-#[stable(feature = "stable_fn", since = "1.0.0")] // ok!
+#[stable(feature = "stable_fn", since = "1.0.0")]
 fn stable_fn() {}
 ```