Add link to EBNF definition

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from #20023)

doc/man7/property.pod

@@ -158,6 +158,9 @@ The lexical syntax in EBNF is given by:
  NumberLiteral  ::= '0' ( [0-7]* | 'x' [0-9A-Fa-f]+ ) | '-'? [1-9] [0-9]+
  PropertyName   ::= [A-Za-z] [A-Za-z0-9_]* ( '.' [A-Za-z] [A-Za-z0-9_]* )*
 
+The flavour of EBNF being used is defined by:
+L<https://www.w3.org/TR/2010/REC-xquery-20101214/#EBNFNotation>.
+
 =head1 HISTORY
 
 Properties were added in OpenSSL 3.0