Added more documentation for vtable_for

Author: izagawd

library/core/src/intrinsics/mod.rs

@@ -2657,8 +2657,7 @@ pub unsafe fn vtable_size(ptr: *const ()) -> usize;
 #[rustc_intrinsic]
 pub unsafe fn vtable_align(ptr: *const ()) -> usize;
 
-/// FIXME: write actual docs (ivarflakstad)
-/// The intrinsic will return the vtable of `t` through the lens of `U`.
+/// The intrinsic returns the vtable of `T` through the lens of `U`.
 ///
 /// # Compile-time failures
 /// Determining whether `T` implements `U` requires trait resolution by the compiler.