Don't generate hyperlinks to entities that are not part of the DTD or the docs.

[Klortho]

The documentor should be smart enough to not attempt to generate hyperlinks to entities in these cases:

• The "big five": < & > ' "
• NCRs, such as ü ...
• Whenever "-e" is not specified, and hence no documentation pages are being generated for entities
• Any entity that is not part of the DTD at all.