Improve documentation of mrf_object_age
See merge request pleroma/pleroma!2305
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#### :mrf_object_age
-* `threshold`: Required age (in seconds) of a post before actions are taken.
+* `threshold`: Required time offset (in seconds) compared to your server clock of an incoming post before actions are taken.
+ e.g., A value of 900 results in any post with a timestamp older than 15 minutes will be acted upon.
* `actions`: A list of actions to apply to the post:
* `:delist` removes the post from public timelines
* `:strip_followers` removes followers from the ActivityPub recipient list, ensuring they won't be delivered to home timelines