DOC: sample: Add a comment in 'check_operator' to explain why 'vars_check_arg' should ignore the 'err' buffer

This extra comment ensure that we do not try to pass an 'err' argument
to 'vars_check_arg' otherwise some warnings will be raised if an
operator is given an integer directly in the configuration file.

src/sample.c

@@ -3155,7 +3155,10 @@ static int check_operator(struct arg *args, struct sample_conv *conv,
 	const char *end;
 	long long int i;
 
-	/* Try to decode a variable. */
+	/* Try to decode a variable. The 'err' variable is intentionnaly left
+	 * NULL since the operators accept an integer as argument in which case
+	 * vars_check_arg call will fail.
+	 */
 	if (vars_check_arg(&args[0], NULL))
 		return 1;