Document Str.subst

Part of #2277 D#2277

Author: zoffixznet

doc/Type/Cool.pod6

@@ -1341,6 +1341,14 @@ Defined as:
 
 Coerces the invocant to L<Stringy> and calls L<Str.match|/type/Str#method_match>.
 
+=head2 method subst
+
+Defined as:
+
+    method subst(|)
+
+Coerces the invocant to L<Stringy> and calls L<Str.subst|/type/Str#method_subst>.
+
 =head2 method trans
 
 Defined as:
@@ -1472,7 +1480,7 @@ the case it's being used as a C<sub>
 
 
 TODO:
-match, subst, sprintf, printf, samecase
+sprintf, printf, samecase
 
 =end comment