core.after: Improve documentation details about how time is handled

doc/lua_api.md

@@ -6922,6 +6922,9 @@ Timing
     * Optional: Variable number of arguments that are passed to `func`
     * Jobs set for earlier times are executed earlier. If multiple jobs expire
       at exactly the same time, then they are executed in registration order.
+    * `time` is a lower bound. The job is executed in the first globalstep that
+      started at least `time` seconds after the current globalstep started.
+      If `time` is `0`, it's executed in the next step.
 
 * `job:cancel()`
     * Cancels the job function from being called