Update HISTORY.md with IO fencing error code (#7402)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7402

Reviewed By: pdillinger

Differential Revision: D23761689

Pulled By: anand1976

fbshipit-source-id: 59e10f0aaa80f6c0f5a46dc99467138c4cee0511

HISTORY.md

@@ -39,6 +39,7 @@
 * Added file_checksum and file_checksum_func_name to TableFileCreationInfo, which can pass the table file checksum information through the OnTableFileCreated callback during flush and compaction.
 * A warning is added to `DB::DeleteFile()` API describing its known problems and deprecation plan.
 * Add a new stats level, i.e. StatsLevel::kExceptTickers (PR7329) to exclude tickers even if application passes a non-null Statistics object.
+* Added a new status code IOStatus::IOFenced() for the Env/FileSystem to indicate that writes from this instance are fenced off. Like any other background error, this error is returned to the user in Put/Merge/Delete/Flush calls and can be checked using Status::IsIOFenced().
 
 ### Behavior Changes
 * File abstraction `FSRandomAccessFile.Prefetch()` default return status is changed from `OK` to `NotSupported`. If the user inherited file doesn't implement prefetch, RocksDB will create internal prefetch buffer to improve read performance.