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

…protocol docs (ArchipelagoMW#2989)

Co-authored-by: Exempt-Medic <60412657+Exempt-Medic@users.noreply.github.com>

docs/network protocol.md

@@ -31,6 +31,9 @@ There are also a number of community-supported libraries available that implemen
 | GameMaker: Studio 2.x+        | [see Discord](https://discord.com/channels/731205301247803413/1166418532519653396)                 |                                                                                 |
 
 ## 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 have already processed in a previous play session.  
+To ensure the client is able to reject those items if it needs to, each item in the packet has 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.