Merge pull request #3341 from dfinity/@anchpop/NF-hotkey-copy

Add section explaining how hotkeys are copied to the SNS for NF participants

docs/developer-docs/daos/nns/neurons-fund.mdx

@@ -44,6 +44,21 @@ Please note: In the Matched Funding framework, the minimum and maximum funding t
 
 Participation in voting on the SNS proposals is passed through to the NF neurons via hotkeys on the SNS neurons owned by the NNS. This means, that NF neurons are owned by NNS but permission is given to the principals that exposed maturity to NF neurons to vote on SNS proposals.
 
+When the swap is finalized, NF participants' neurons' hotkeys are copied to the SNS neuron that is created for them. This is convenient for NF participants who want to keep their neurons' controller key in cold storage, and want to vote on SNS proposals. These users can add a hotkey to their NNS neuron, and it will automatically be copied to their SNS neuron for new SNSes. 
+
+:::info
+Note that at most 3 hotkeys are copied. If your NNS neuron has more hotkeys, 3 of them will be chosen, prioritizing self-authenticating principals (i.e. non-canister principals) and favoring principals added earlier.
+
+This means that users can influence which hotkeys are selected by removing and then re-adding the hotkeys they don't want to be copied.
+
+If the user has hotkeys = [A, B, C, D] (all self-authenticating), and they want [A, B, D] to be copied to the SNS, then they should do something like:
+
+1. hotkeys.remove(C) // hotkeys = [A, B, D]
+2. hotkeys.add(C)  // hotkeys = [A, B, D, C]
+
+Now [A, B, D] would be copied.
+:::
+
 The voting power of the SNS neurons is proportional to the amount of maturity exposed.
 
 ### 4: Increasing maturity of NF neurons