Document the proper way to concat arrays. (#1617)

In 5to6 - it was not properly documented. Also spell "," as the comma.

This got me tripped when working on perl6-examples.

Author: shlomif

doc/Language/5to6-nutshell.pod6

@@ -567,7 +567,6 @@ for full details on all operators.
 
 Unchanged:
 
-=item C<,> List Separator
 =item C<+> Numeric Addition
 =item C<-> Numeric Subtraction
 =item C<*> Numeric Multiplication
@@ -581,6 +580,18 @@ Unchanged:
 =item C«== != < > <= >=»   Numeric comparisons
 =item C<eq ne lt gt le ge>  String comparisons
 
+=head2 C<,> (Comma) List Separator
+
+Unchanged, but note that in order to flatten an array variable to a list (in
+order to append or prefix more items) one should use the C<|> operator. E.g:
+
+=for code
+    my @numbers = (100, 200, 300);
+    my @more_numbers = (500, 600, 700);
+    my @all_numbers = [|@numbers, 400, |@more_numbers];
+
+That way one can concatenate arrays.
+
 =head2 C«<=> cmp» Three-way comparisons
 
 In Perl 5, these operators returned -1, 0, or 1.