Improve documentation for error code checking

Improve comments explaining error code checking, fix incorrect comments
and make a small formatting fix.

Signed-off-by: Chris Jones <christopher.jones@arm.com>
diff --git a/include/mbedtls/error.h b/include/mbedtls/error.h
index 3d8a5ea..52b8188 100644
--- a/include/mbedtls/error.h
+++ b/include/mbedtls/error.h
@@ -122,7 +122,7 @@
 /**
  * \brief Combines a high-level and low-level error code together.
  *
- *        Wrapper function for mbedtls_err_add_ext(). See that function for
+ *        Wrapper macro for mbedtls_error_add_ext(). See that function for
  *        more details.
  */
 #define MBEDTLS_ERROR_ADD( high, low ) \