Clarify return of Str.subst-mutate with multi-match options

Rakudo fix: rakudo/rakudo@77794a28aa
            rakudo/rakudo@e0d39321e1
Spec:       Raku/roast@c537763529
            Raku/roast@8c358cfc87

Author: zoffixznet

doc/Type/Str.pod6

@@ -911,9 +911,10 @@ regex, not the C<subst> method call.
 Where L<subst> returns the modified string and leaves the original
 unchanged, it is possible to mutate the original string by using
 C<subst-mutate>. If the match is successful, the method returns a L<Match>
-object representing the successful match; if C<:g> (or C<:global>) argument
-is used, returns a L<List> of L<Match> objects. If no matches happen,
-returns L<Nil>.
+object representing the successful match, otherwise returns L<Nil>. If C<:nth>
+(or one of its aliases) with L<Iterable> value, C<:g>, C<:global>, or C<:x>
+arguments are used, returns a L<List> of L<Match> objects, or an empty L<List>
+if no matches occured.
 
     my $some-string = "Some foo";
     my $match = $some-string.subst-mutate(/foo/, "string");