mentioning Proxy in Str.substr-rw

doc/Type/Str.pod6

@@ -682,10 +682,10 @@ Also using a different value or an incorrect starting index won't match:
 
     method substr-rw($from, $length?)
 
-A version of C<substr> that returns a writable reference to a part of a
-string variable.  Its first argument, C<$from> specifies the index in the
-string from which a substitution should occur, and its last argument,
-C<$length> specifies how many characters are to be replaced.
+A version of C<substr> that returns a L<Proxy|/type/Proxy> functioning as a
+writable reference to a part of a string variable. Its first argument, C<$from>
+specifies the index in the string from which a substitution should occur, and
+its last argument, C<$length> specifies how many characters are to be replaced.
 
 For example, in its method form, if one wants to take the string C<"abc">
 and replace the second character (at index 1) with the letter C<"z">, then