Renamed "downcast_trait" to "try_as_dyn", added documentation, including recursion limit docs

Author: izagawd

library/core/src/intrinsics/mod.rs

@@ -2761,6 +2761,11 @@ pub unsafe fn vtable_align(ptr: *const ()) -> usize;
 /// In some cases, that resolution can exceed the recursion limit,
 /// and compilation will fail instead of this function returning `None`.
 ///
+/// # Compile-time failures
+/// Determining whether `T` implements `U` requires trait resolution by the compiler.
+/// In some cases, that resolution can exceed the recursion limit,
+/// and compilation will fail *instead of* this function returning `None`.
+///
 /// # Safety
 ///
 /// `ptr` must point to a vtable.