Summary: "Batch" is ambiguous in this context. It can mean "write batch" or commit group. Change it to commit group to be clear.

Test Plan: Build

Reviewers: MarkCallaghan, yhchiang, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D62055