RelNotes: spelling & grammar tweaks

Signed-off-by: Marc Branchaud <marcnarc@xiplink.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git 2.2

Signed-off-by: Junio C Hamano <gitster@pobox.com>
external odb work in progress
external odb foreach