message: [doc] Add comment for m_headers_cache
diff: @@ -39,6 +39,7 @@ private:
  size_t WriteFilterToDisk(FlatFilePos& pos, const BlockFilter& filter);
  Mutex m_cs_headers_cache;
  + /** cache of block hash to filter header, to avoid disk access when responding to getcfcheckpt. */
  std::unordered_map<uint256, uint256, FilterHeaderHasher> m_headers_cache GUARDED_BY(m_cs_headers_cache);
  protected:
  