Document bits of fancy string expander
Reorganize OOExpandKey* fast path

* Key expansion path is now a flag to OOExpandDescriptionString()
* OOExpandKeyWithSeed() is now a thin wrapper
* Added OOExpandFancyKey() macro
Replace OOExpand[Key]() with "fancy" equivalents

The only performance impact is that [UNIVERSE systemSeed] is called at each
call site instead of in a wrapper function.