Add extra note in docstring about terminating null character

Signed-off-by: Emerson Knapp <emerson.b.knapp@gmail.com>

rcl/include/rcl/type_version_hash.h

@@ -39,7 +39,9 @@ extern "C"
  *
  * \param[in] type_description Prefilled TypeDescription message to be translated
  * \param[out] output_repr An initialized empty char array that will be filled with
- *   the JSON representation of type_description
+ *   the JSON representation of type_description. Note that output_repr will have a
+ *   terminating null character, which should be omitted from hashing. To do so, use
+ *   (output_repr.buffer_length - 1) or strlen(output_repr.buffer) for the size of data to hash.
  * \return RCL_RET_OK on success, RCL_RET_ERROR if any problems occur in translation
  */
 RCL_PUBLIC