doc [&sub-name] form for meta ops

doc/Language/operators.pod

@@ -109,11 +109,12 @@ L<Defining Operators functions|/language/functions#Defining_Operators>.
 
 =head1 Meta Operators
 
-Meta operators can be parameterized with other operators in the same way as
-functions can take functions as parameters. Perl 6 will generate the actual
-combined operator in the background, allowing the mechanism to be applied
-to user defined operators. There are quite a few Meta operators with
-different semantics, as explained in detail as follows.
+Meta operators can be parameterized with other operators or subroutines in the
+same way as functions can take functions as parameters. To use a subroutine as
+a parameter prefix it's name with a C<&>. Perl 6 will generate the actual
+combined operator in the background, allowing the mechanism to be applied to
+user defined operators. There are quite a few Meta operators with different
+semantics, as explained in detail as follows.
 
 =head1 Substitution Operators
 
@@ -298,6 +299,8 @@ Reduction operators apply any infix operator, surrounded by C<[> and C<]>,
 element by element and return the resulting value.
 
     say [+] 1, 2, 3; # 6
+    sub plus { $^a + $^b };
+    say [&plus] 1, 2, 3; # 6
 
 They can be defined as a list prefix operators or will be generated automatically.