man: Add a reference to deb-symbols(5) in deb-src-symbols(5) DESCRIPTION

Makes it easier to see what this format is based on.

Prompted-by: #970083

man/deb-src-symbols.pod

@@ -33,7 +33,8 @@ B<debian/symbols>
 =head1 DESCRIPTION
 
 The symbol file templates are shipped in Debian source packages, and its
-format is a superset of the symbols files shipped in binary packages.
+format is a superset of the symbols files shipped in binary packages,
+see L<deb-symbols(5)>.
 
 =head2 Comments