Adds examples using backreferences in regexes

This closes #1545. Since this part is going to probably be moved dur
to #2890, it will move with them; this seemed the most reasonable part
to document these back references, since it's where they are actually used.

doc/Language/regexes.pod6

@@ -1108,6 +1108,7 @@ assigned to other variables or used for subsequent matches
 =for code
 say "11" ~~ /(\d) {} $0/; # OUTPUT: «｢11｣␤ 0 => ｢1｣␤»
 
+
 X<|:my>
 C<:my> helps scoping the C<$c> variable within the regex and beyond; in this
 case we can use it in the next sentence to show what has been matched inside the
@@ -1117,6 +1118,22 @@ regex. This can be used for debugging inside regular expressions, for instance:
     $paragraph ~~ rx| :my $counter = 0; ( \V* { ++$counter } ) *%% \n |;
     say "Matched $counter lines"; # OUTPUT: «Matched 3 lines␤»
 
+Since C<:my> blocks are simply declarations, the match variable C<$/> or
+numbered matches such as C<$0> will not be available in them unless they are
+previously I<published> by inserting the empty block (or any block):
+
+=for code
+"aba" ~~ / (a) b {} :my $c = $/; /;
+say $c; # OUTPUT: «｢ab｣␤ 0 => ｢a｣␤»
+
+Any other code block will also reveal the variables and make them available in
+declarations:
+
+=for code
+"aba" ~~ / (a) {say "Check so far ", ~$/} b :my $c = ~$0; /;
+# OUTPUT: «Check so far a␤»
+say "Capture $c"; # OUTPUT: «Capture a␤»
+
 X<|:our>
 The C<:our>, similarly to L<C<our>|/syntax/our> in classes, can be used in
 L<Grammar|/type/Grammar>s to declare variables that can be accessed, via its