Refactoring around TyCon.isSynTyCon

* Document isSynTyCon better
* Add isTypeSyonymTyCon for regular H98 type synonyms
* Use isTypeSynonymTyCon rather than isSynTyCon where
  the former is really intended

All arose as part of a bug I introduced when fixing Trac #9102,
thinking that isSynTyCon meant H98 type syononyms.