Note regex_sets is a synonym

Fixes #23232

"Extended Bracketed Character Classes" is the formal name documented;
but "regex_sets" is what you specify to 'feature' to enable/disable it.
Add a bit of text so that if you know one name, you can find the other.

Author: khwilliamson

pod/perlrecharclass.pod

@@ -1027,8 +1027,11 @@ either construct raises an exception.
 
 =head3 Extended Bracketed Character Classes
 X<character class>
+X<regex_sets>
 X<set operations>
 
+(Also called C<regex_sets>.)
+
 This is a fancy bracketed character class that can be used for more
 readable and less error-prone classes, and to perform set operations,
 such as intersection. An example is