Apply suggestions from code review

Co-authored-by: Jon Skeet <[email protected]>

standard/statements.md

@@ -1111,7 +1111,7 @@ is then equivalent to:
  {
  while (e.MoveNext())
  {
- ref V v = ref (V)(T)e.Current;
+ ref V v = ref e.Current;
  «embedded_statement»
  }
  }
@@ -1124,6 +1124,8 @@ is then equivalent to:
 
 The variable `e` is not visible or accessible to the expression `x` or the embedded statement or any other source code of the program. The reference variable `v` is read-write in the embedded statement, but `v` shall not be ref-reassigned (§12.21.3). If there is not an identity conversion (§10.2.2) from `T` (the iteration type) to `V` (the *local_variable_type* in the `foreach` statement), an error is produced and no further steps are taken.
 
+A `foreach` statement of the form `foreach (ref readonly V v in x) «embedded_statement»` has a similar equivalent form, but the reference variable `v` is `ref readonly` in the embedded statement, and therefore cannot be ref-reassigned or reassigned.
+
 > *Note*: If `x` has the value `null`, a `System.NullReferenceException` is thrown at run-time. *end note*
 
 An implementation is permitted to implement a given *foreach_statement* differently; e.g., for performance reasons, as long as the behavior is consistent with the above expansion.