fix and comment on test

tests/system_resource_util_test.c

@@ -11,16 +11,24 @@
 static int s_test_memory_usage_maxrss(struct aws_allocator *allocator, void *ctx) {
     (void)ctx;
 
+    /*
+     * Note: mem usage apis currently rely on getrusage on posix systems. 
+     * On freebsd maxrss seems to return current process rss based on testing,
+     * while on every other posix platform maxrss is high water mark for rss
+     * over the program lifetime.
+     * Workaround it by allocating a buffer first. Long term using procfs should
+     * avoid the issue.
+    */
     struct aws_byte_buf temp;
     aws_byte_buf_init(&temp, allocator, 8 * 1024 * 1024);
 
-    aws_byte_buf_clean_up_secure(&temp);
-
     struct aws_memory_usage_stats mu;
     ASSERT_SUCCESS(aws_init_memory_usage_for_current_process(&mu));
 
     ASSERT_TRUE(mu.maxrss > 0);
 
+     aws_byte_buf_clean_up_secure(&temp);
+
     return 0;
 }