add comment in SuperVersion referencing logic

Summary:
The referencing logic is super confusing so added a comment at the part that took me longest to figure out.
Closes https://github.com/facebook/rocksdb/pull/2996

Differential Revision: D6034969

Pulled By: ajkr

fbshipit-source-id: 9cc2e744c1f79d6d57d378f86ed59238a5f583db

db/column_family.cc

@@ -854,6 +854,10 @@ SuperVersion* ColumnFamilyData::GetReferencedSuperVersion(
   sv = GetThreadLocalSuperVersion(db_mutex);
   sv->Ref();
   if (!ReturnThreadLocalSuperVersion(sv)) {
+    // This Unref() corresponds to the Ref() in GetThreadLocalSuperVersion()
+    // when the thread-local pointer was populated. So, the Ref() earlier in
+    // this function still prevents the returned SuperVersion* from being
+    // deleted out from under the caller.
     sv->Unref();
   }
   return sv;