Docs: Mention the "last received item index" paradigm in the network protocol docs

docs/network protocol.md

@@ -31,6 +31,9 @@ There are also a number of community-supported libraries available that implemen
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 ## Synchronizing Items
+After a client connects, it will receive all previously collected items for its associated slot in a [ReceivedItems](#ReceivedItems) packet. This will include items the client may already have processed in a previous play session.
+To make it possible to reject those items if the client shouldn't process them again, each item in the packet will have an associated `index` argument. You will need to find a way to save the "last processed item index" to the player's local savegame, a local file, or something to that effect. Before connecting, you should load that "last processed item index" value and compare against it in your received items handling.
+
 When the client receives a [ReceivedItems](#ReceivedItems) packet, if the `index` argument does not match the next index that the client expects then it is expected that the client will re-sync items with the server. This can be accomplished by sending the server a [Sync](#Sync) packet and then a [LocationChecks](#LocationChecks) packet.
 
 Even if the client detects a desync, it can still accept the items provided in this packet to prevent gameplay interruption.