have better explanation for relate_types

compiler/rustc_middle/src/mir/syntax.rs

@@ -1077,9 +1077,12 @@ pub enum ProjectionElem<V, T> {
     OpaqueCast(T),
 
     /// A `Subtype(T)` projection is applied to any `StatementKind::Assign` where
-    /// type of lvalue doesn't match type of rvalue, primary goal being making subtyping
+    /// type of lvalue doesn't match the type of rvalue, the primary goal is making subtyping
     /// explicit during optimizations and codegen.
     ///
+    /// This projection doesn't impact the runtime behavior of the program except for potentially changing
+    /// some type metadata of the interpreter or codegen backend.
+    ///
     /// This goal is achieved with mir_transform pass `Subtyper`, which runs right after
     /// borrowchecker, as we only care about subtyping that can affect trait selection and
     /// `TypeId`.