Add section in 'traps' about $/ changing on every match.

See issue #1752

Author: fluca1978

doc/Language/traps.pod6

@@ -743,6 +743,12 @@ say 42 ~~ / [ 42 ||  0 ] | 4/; # OUTPUT: «｢42｣␤»
 The code above may seem like it is producing a wrong result, but the
 implementation is actually right.
 
+=head2 C<$/> changes each time a regular expression is matched
+
+Eash time a regular expression is matched against something, the special
+variable C<$/> holding the result L<Match object|/type/Match>
+is changed accordingly to the result of the match (that could also be C<Nil>).
+
 =head1 Common Precedence Mistakes
 
 =head2 Adverbs and Precedence