Summary:
Add comment to DisableManualCompaction() which was missing.
Also explictly return from DBImpl::CompactRange() to avoid memtable flush when manual compaction is disabled.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/8186
Test Plan: Run existing unit tests.
Reviewed By: jay-zhuang
Differential Revision: D27744517
fbshipit-source-id: 449548a48905903b888dc9612bd17480f6596a71