add example

doc/lua_api.md

@@ -6925,6 +6925,11 @@ Timing
     * `time` is a lower bound. The job is executed in the first server-step that
       started at least `time` seconds after the last time a server-step started,
       measured with globalstep dtime.
+      In particular this can result in relatively large delays if `time` is close
+      to the server-step dtime. For example, with a target server-step of 0.09 s
+      (the engine uses the target as a lower bound for the actual server-step
+      dtime), `minetest.after(0.09, ...)` usually waits two steps, resulting in
+      a delay of about 0.18 s.
     * If `time` is `0`, the job is executed in the next step.
 
 * `job:cancel()`