hs-v3: Improve documentation of hs_cache_client_descriptor_t

   * using the next blinded key of the service. */

  time_t expiration_ts;

  /** The cached descriptor, this object is the owner. It can't be NULL. A

   * cache object without a valid descriptor is not possible. */

  /** The cached decoded descriptor, this object is the owner. This can be

   * NULL if the descriptor couldn't be decoded due to missing or bad client

   * authorization. It can be decoded later from the encoded_desc object if

   * the proper client authorization is given tor. */

  hs_descriptor_t *desc;

  /** Encoded descriptor in string form. Can't be NULL. */