Add rustdoc test for #[rustc_must_implement_one_of]

src/test/rustdoc/must_implement_one_of.rs

@@ -0,0 +1,10 @@
+#![crate_name = "c"]
+#![feature(rustc_attrs)]
+
+#[rustc_must_implement_one_of(a, b)]
+// @matches c/trait.Trait.html '//*[@class="stab must_implement"]' \
+//      'At least one of a, b methods is required.$'
+pub trait Trait {
+    fn a() {}
+    fn b() {}
+}