Mention the consistency check improvement in HISTORY.md (#6924)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/6924

Reviewed By: cheng-chang

Differential Revision: D21865662

Pulled By: ltamasi

fbshipit-source-id: 83a01bcbb779cfba941154a36a9e735293a93211

HISTORY.md

@@ -17,6 +17,7 @@
 * BlobDB now explicitly disallows using the default column family's storage directories as blob directory.
 * DeleteRange now returns `Status::InvalidArgument` if the range's end key comes before its start key according to the user comparator. Previously the behavior was undefined.
 * ldb now uses options.force_consistency_checks = true by default and "--disable_consistency_checks" is added to disable it.
+* The consistency checks that validate LSM state changes (table file additions/deletions during flushes and compactions) are now stricter, more efficient, and no longer optional, i.e. they are performed even if `force_consistency_checks` is `false`.
 
 ### New Feature
 * sst_dump to add a new --readahead_size argument. Users can specify read size when scanning the data. Sst_dump also tries to prefetch tail part of the SST files so usually some number of I/Os are saved there too.