.subst: give examples on how to use captures

Hopefully this will help others out, as it is somewhat counterintuitive.

Author: samcv

doc/Type/Str.pod6

@@ -813,16 +813,29 @@ C<s/matcher/replacement/>.
 C<$matcher> can be a L<Regex>, or a literal C<Str>. Non-Str matcher arguments
 of type L<Cool> are coerced to C<Str> for literal matching.
 
+=head3 Literal replacement substitution
+
     my $some-string = "Some foo";
     my $another-string = $some-string.subst(/foo/, "string"); # gives 'Some string'
     $some-string.=subst(/foo/, "string"); # in-place substitution. $some-string is now 'Some string'
 
+=head3 Closures
+
 The replacement can be a closure:
 
     my $i = 41;
     my $str = "The answer is secret.";
     my $real-answer = $str.subst(/secret/, {++$i}); # The answer to everything
 
+=head3 Captures
+
+If you are going to use captures, you have to use a closure like so:
+
+    say 'abc123defg'.subst(/(\d+)/, { " before $0 after " })
+    # OUTPUT: «abc before 123 after defg␤»
+
+=head3 More Examples
+
 Here are other examples of usage:
 
     my $str = "Hey foo foo foo";
@@ -840,6 +853,8 @@ The C<:nth> adverb has readable English-looking variants:
     say 'ooooo'.subst: 'o', 'x', :3rd; # OUTPUT: «ooxoo␤»
     say 'ooooo'.subst: 'o', 'x', :4th; # OUTPUT: «oooxo␤»
 
+=head3 Adverbs
+
 The following adverbs are supported
 
 =begin table