Add "UB" to glossary

Author: zoffixznet

doc/Language/glossary.pod6

@@ -1061,6 +1061,11 @@ X<|value>
 
 A value is what is actually contained in a container such as a variable. Used in expressions such as L<lvalue>, to indicate that that particular container can be assigned to.
 
+=head1 UB
+X<|UB>
+
+Stands for "Undefined Behaviour". In other words, it is something that is not explicitly specified by the language specification.
+
 =head1 Value type
 X<|Value type>