Mitigate focused memory leaks in core doctests for Miri.

If/when `-Zmiri-disable-leak-check` is able to be used at test-granularity, it should applied to these tests instead of unleaking.

core/src/mem/manually_drop.rs

@@ -62,6 +62,9 @@ impl<T> ManuallyDrop<T> {
     /// x.truncate(5); // You can still safely operate on the value
     /// assert_eq!(*x, "Hello");
     /// // But `Drop` will not be run here
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # let _ = ManuallyDrop::into_inner(x);
     /// ```
     #[must_use = "if you don't need the wrapper, you can use `mem::forget` instead"]
     #[stable(feature = "manually_drop", since = "1.20.0")]

core/src/mem/maybe_uninit.rs

@@ -448,6 +448,9 @@ impl<T> MaybeUninit<T> {
     /// let mut x = MaybeUninit::<String>::uninit();
     ///
     /// x.write("Hello".to_string());
+    /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
+    /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
+    /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
     /// // This leaks the contained string:
     /// x.write("hello".to_string());
     /// // x is initialized now: