Document user-facing changes related to WT index format change

WT Index format change BSON>KeyIndex implies, from Mathias:

@Docs: Two things that need mention: upgrading WT data from older RCs will require --repair. Also, when doing a covered index query, if the result is a double with the value NaN, we no longer guarantee which NaN you get back.

Former, in compatibility notes should have limited impact.
Latter, at http://docs.mongodb.org/manual/core/query-optimization/#limitations and probably also in compatibility notes and in WT 'blurbs'