Document using C<|> or C<||> as a trap

#1141

Author: tisonkun

doc/Language/traps.pod6

@@ -588,6 +588,24 @@ without L<MONKEY-SEE-NO-EVAL|
 In theory, careless use of C«<{}>» will only result in an exception being
 thrown, and should not introduce security issues.
 
+=head2 C<|> vs C<||>: which branch will win
+
+To match one of several possible alternatives, C<||> or C<|> will be used. But
+they are so different.
+
+While there are more alternations than one matched, for those separated by
+C<||>, the first matching alternation wins; for those separated by C<|>,
+which to win is decided by LTM strategy. See also:
+L<documentation on C<||>|/language/regexes#Alternation:_||> and
+L<documentation on C<|>|/language/regexes#Longest_Alternation:_|>.
+
+For advice of which to use: for simple regexes just using C<||> instead of C<|>
+will get you familiar semantics, but if writing grammars then it's useful to
+learn about LTM and declarative prefixes and prefer C<|>. And keep yourself
+away from using them in one regex. When you have to do that, add parentheses
+and ensure that you know how LTM strategy works to make the code
+do what you want.
+
 =head1 Common Precedence Mistakes
 
 =head2 Adverbs and Precedence