Update regexes.pod6

1. Remove `=head1 Look-around assertions` and put `Lookahead/Lookbehind assertions` into `Anchor`

2. Add example to show difference between C«<|w>» and C<«>, C<»>:

Author: tisonkun

doc/Language/regexes.pod6

@@ -741,9 +741,14 @@ You can also use the variants C<«> and C<»> :
     say so $str ~~ /« own/;          # OUTPUT: «False␤»
     say so $str ~~ /own »/;          # OUTPUT: «True␤»
 
-=head1 Look-around assertions
+To see the difference between C«<|w>» and C<«>, C<»>:
 
-=head2 X<Lookahead assertions|regex,before>
+    say "stuff here!!!".subst(:g, />>/, '|');   # OUTPUT: «stuff| here|!!!␤»
+    say "stuff here!!!".subst(:g, /<</, '|');   # OUTPUT: «|stuff |here!!!␤»
+    say "stuff here!!!".subst(:g, /<|w>/, '|'); # OUTPUT: «|stuff| |here|!!!␤»
+
+
+=head2 X<Lookahead assertions C«<?before pattern>»|regex,before>
 
 To check that a pattern appears before another pattern, use a
 lookahead assertion via the C<before> assertion. This has the form:
@@ -770,7 +775,7 @@ match with
 
     say "foobar" ~~ rx{ foo <!before bar> }   # OUTPUT: «Nil␤»
 
-=head2 X<Lookbehind assertions|regex,after>
+=head2 X<Lookbehind assertions C«<?after pattern>»|regex,after>
 
 To check that a pattern appears before another pattern, use a
 lookbehind assertion via the C<after> assertion. This has the form: