It's sometimes mentioned in the notes but only as a possible outcome of some the tehcniques you mention. Would this deserve its own bullet?
Topic on User talk:TJones (WMF)/Notes/Potential Applications of Natural Language Processing to On-Wiki Search
David and I talked about this and I should make it clearer that the order and the groupings aren't about what's more important or what we should work on. "Spelling correction" is under "Query rewriting" just a way for me to organize all the information.