Make "unneccesary visibility qualifier" error much more clear

compiler/rustc_error_codes/src/error_codes/E0449.md

@@ -1,4 +1,6 @@
-A visibility qualifier was used when it was unnecessary.
+A visibility qualifier was used where one is not permitted. Visibility
+qualifiers are not permitted on enum variants, trait items, impl blocks, and
+extern blocks, as they already share the visibility of the parent item.
 
 Erroneous code examples:
 
@@ -9,15 +11,18 @@ trait Foo {
     fn foo();
 }
 
-pub impl Bar {} // error: unnecessary visibility qualifier
+enum Baz {
+    pub Qux, // error: visibility qualifiers are not permitted here
+}
 
-pub impl Foo for Bar { // error: unnecessary visibility qualifier
-    pub fn foo() {} // error: unnecessary visibility qualifier
+pub impl Bar {} // error: visibility qualifiers are not permitted here
+
+pub impl Foo for Bar { // error: visibility qualifiers are not permitted here
+    pub fn foo() {} // error: visibility qualifiers are not permitted here
 }
 ```
 
-To fix this error, please remove the visibility qualifier when it is not
-required. Example:
+To fix this error, simply remove the visibility qualifier. Example:
 
 ```
 struct Bar;
@@ -26,12 +31,18 @@ trait Foo {
     fn foo();
 }
 
+enum Baz {
+    // Enum variants share the visibility of the enum they are in, so
+    // `pub` is not allowed here
+    Qux,
+}
+
 // Directly implemented methods share the visibility of the type itself,
-// so `pub` is unnecessary here
+// so `pub` is not allowed here
 impl Bar {}
 
-// Trait methods share the visibility of the trait, so `pub` is
-// unnecessary in either case
+// Trait methods share the visibility of the trait, so `pub` is not
+// allowed in either case
 impl Foo for Bar {
     fn foo() {}
 }