Remove --display-doctest-warnings

This can be replicated in full with other existing features, there's no
need to have a separate option for it.

This also fixes a bug where `--test-args=--show-output` had no effect,
and updates the documentation.

src/test/rustdoc-ui/display-output.rs

@@ -1,9 +1,15 @@
+// Test that `--show-output` has an effect and `allow(unused)` can be overriden.
+
 // check-pass
-// compile-flags:-Zunstable-options --display-doctest-warnings --test
+// edition:2018
+// compile-flags:--test --test-args=--show-output
 // normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
 // normalize-stdout-test "finished in \d+\.\d+s" -> "finished in $$TIME"
 
 /// ```
+/// #![warn(unused)]
 /// let x = 12;
+///
+/// fn foo(x: &std::fmt::Display) {}
 /// ```
 pub fn foo() {}