Add test description

Author: oli-obk

tests/ui/privacy/ufc-method-call.different_name.stderr

@@ -1,5 +1,5 @@
 error[E0599]: no function or associated item named `foo` found for struct `Foo<B>` in the current scope
-  --> $DIR/ufc-method-call.rs:21:27
+  --> $DIR/ufc-method-call.rs:27:27
    |
 LL |     pub struct Foo<T>(T);
    |     ----------------- function or associated item `foo` not found for this struct

tests/ui/privacy/ufc-method-call.rs

@@ -1,3 +1,9 @@
+//! This test used to report that the method call cannot
+//! call the private method `Foo<A>::foo`, even though the user
+//! explicitly selected `Foo<B>::foo`. This is because we only
+//! looked for methods of the right name, without properly checking
+//! the `Self` type
+
 //@ revisions: same_name different_name
 
 pub mod test {

tests/ui/privacy/ufc-method-call.same_name.stderr

@@ -1,5 +1,5 @@
 error[E0624]: associated function `foo` is private
-  --> $DIR/ufc-method-call.rs:21:27
+  --> $DIR/ufc-method-call.rs:27:27
    |
 LL |         fn foo() {}
    |         -------- private associated function defined here