Document memtable flush behavior in CancelAllBackgroundWork()

Summary:
Update History.md to reflect recent change that ensures unpersisted data
is flushed even if clients call CancelAllBackgroundWork() directly.

Test Plan: Review rendering of markdown.

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D62703

HISTORY.md

 # Rocksdb Change Log
 ## Unreleased
+### Public API Change
+* CancelAllBackgroundWork() flushes all memtables for databases containing writes that have bypassed the WAL (writes issued with WriteOptions::disableWAL=true) before shutting down background threads.
+
 ### New Features
 * Introduce NewClockCache, which is based on CLOCK algorithm with better concurrent performance in some cases. It can be used to replace the default LRU-based block cache and table cache. To use it, RocksDB need to be linked with TBB lib.