Fix old comment about HyperClockCache and SecondaryCache (#11517)

Summary:
Support was added in 8.1.0

Pull Request resolved: #11517

Test Plan: comments only

Reviewed By: anand1976

Differential Revision: D46489929

Pulled By: pdillinger

fbshipit-source-id: 4fd30078389065c9ec225bf55b6773f1641f0646

include/rocksdb/cache.h

@@ -374,7 +374,6 @@ inline std::shared_ptr<SecondaryCache> NewCompressedSecondaryCache(
 // * Requires an extra tuning parameter: see estimated_entry_charge below.
 // Similarly, substantially changing the capacity with SetCapacity could
 // harm efficiency.
-// * SecondaryCache is not yet supported.
 // * Cache priorities are less aggressively enforced, which could cause
 // cache dilution from long range scans (unless they use fill_cache=false).
 // * Can be worse for small caches, because if almost all of a cache shard is