Document Cool.substr-rw/&substr-rw

Part of #2277 R#2277

doc/Type/Cool.pod6

@@ -1053,6 +1053,17 @@ Coerces the invocant (or in the sub form, the first argument) to
 L<Str|/type/Str>, and calls L<Str.substr|/type/Str#routine_substr> with
 the arguments.
 
+=head2 routine substr-rw
+
+Defined as:
+
+    multi method substr-rw(|) is rw
+    multi sub substr-rw(|) is rw
+
+Coerces the invocant (or in the sub form, the first argument) to
+L<Str|/type/Str>, and calls L<Str.substr-rw|/type/Str#routine_substr-rw> with
+the arguments.
+
 =head2 routine ords
 
 Defined as:
@@ -1445,7 +1456,7 @@ the case it's being used as a C<sub>
 
 
 TODO:
-fmt, substr-rw, trans,
+trans,
 match, subst, sprintf, printf, samecase
 
 =end comment