Mention a new BlobDB option in a couple of comments (#9038)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/9038

Reviewed By: riversand963

Differential Revision: D31663906

fbshipit-source-id: a84ff07a1239f0c2eaeb3b776620fd0f7d4331bc

include/rocksdb/advanced_options.h

@@ -787,7 +787,8 @@ struct AdvancedColumnFamilyOptions {
   // amplification for large-value use cases at the cost of introducing a level
   // of indirection for reads. See also the options min_blob_size,
   // blob_file_size, blob_compression_type, enable_blob_garbage_collection,
-  // and blob_garbage_collection_age_cutoff below.
+  // blob_garbage_collection_age_cutoff, and
+  // blob_garbage_collection_force_threshold below.
   //
   // Default: false
   //
@@ -828,7 +829,8 @@ struct AdvancedColumnFamilyOptions {
   // compaction. Valid blobs residing in blob files older than a cutoff get
   // relocated to new files as they are encountered during compaction, which
   // makes it possible to clean up blob files once they contain nothing but
-  // obsolete/garbage blobs. See also blob_garbage_collection_age_cutoff below.
+  // obsolete/garbage blobs. See also blob_garbage_collection_age_cutoff and
+  // blob_garbage_collection_force_threshold below.
   //
   // Default: false
   //