diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index cff22ea..2fb86c7 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -128,28 +128,32 @@
 void mbedtls_strerror( int errnum, char *buffer, size_t buflen );
 
 /**
- * \brief Translate high level part of a mbed TLS error code into a string
+ * \brief Translate the high-level part of an Mbed TLS error code into a string
  *        representation.
  *
  * This function returns a const pointer to an un-modifiable string. The caller
- * must not try to modify the string and use it only for logging purposes.
+ * must not try to modify the string. It is intended to be used mostly for
+ * logging purposes.
  *
  * \param error_code    error code
  *
- * \return The string representation of the error code.
+ * \return The string representation of the error code, or \c NULL if the error
+ *         code is unknown.
  */
 const char * mbedtls_high_level_strerr( int error_code );
 
 /**
- * \brief Translate low level part of a mbed TLS error code into a string
+ * \brief Translate the low-level part of an Mbed TLS error code into a string
  *        representation.
  *
  * This function returns a const pointer to an un-modifiable string. The caller
- * must not try to modify the string and use it only for logging purposes.
+ * must not try to modify the string. It is intended to be used mostly for
+ * logging purposes.
  *
  * \param error_code    error code
  *
- * \return The string representation of the error code.
+ * \return The string representation of the error code, or \c NULL if the error
+ *         code is unknown.
  */
 const char * mbedtls_low_level_strerr( int error_code );