Add sub_types docs

Author: spastorino

compiler/rustc_mir/src/borrow_check/type_check/mod.rs

@@ -1119,6 +1119,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
         )
     }
 
+    /// Try to relate `sub <: sup`
     fn sub_types(
         &mut self,
         sub: Ty<'tcx>,