2.15 release notes: adjust statement on account data not in NoteDb
2.15 release notes: adjust statement on account data not in NoteDb

Groups are not stored in NoteDb for 2.15 as mentioned on the mailing list:
https://groups.google.com/d/msg/repo-discuss/ur6w4bou21s/RX846hu3AAAJ

(This is not valid for the master branch.)

Change-Id: Ic31a7bd63bdb4ef8e45f09b59fecd011aa8b190a

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