Put “P5” back to glossary

<lizmat> AlexDaniel: I think the term P5 needs to stay in the
         glossary, as it continues to live in X::Syntax::P5 and
         P5Regex

Author: AlexDaniel

doc/Language/glossary.pod6

@@ -505,6 +505,11 @@ X<|PR>
 
 See L<#Pull Request>.
 
+=head2 P5
+X<|P5>
+
+Perl 5
+
 =head2 P6
 X<|P6>