Suggest standalone doctest for non-local impl defs

compiler/rustc_lint/messages.ftl

@@ -548,6 +548,7 @@ lint_non_local_definitions_impl = non-local `impl` definition, `impl` blocks sho
     .without_trait = methods and associated constants are still usable outside the current expression, only `impl Local` and `impl dyn Local` can ever be private, and only if the type is nested in the same item as the `impl`
     .with_trait = an `impl` is never scoped, even when it is nested inside an item, as it may impact type checking outside of that item, which can be the case if neither the trait or the self type are at the same nesting level as the `impl`
     .bounds = `impl` may be usable in bounds, etc. from outside the expression, which might e.g. make something constructible that previously wasn't, because it's still on a publicly-visible type
+    .doctest = make this doc-test a standalone test with its own `fn main() {"{"} ... {"}"}`
     .exception = items in an anonymous const item (`const _: () = {"{"} ... {"}"}`) are treated as in the same scope as the anonymous const's declaration
     .const_anon = use a const-anon item to suppress this lint