Social SDK: Add data migration details for merge/unmerge operations

Document what data transfers during account merging and unmerging
operations, including friends, lobby memberships, and DM message
limitations.

Author: markmandel

docs/discord-social-sdk/development-guides/using-provisional-accounts.mdx

@@ -222,7 +222,7 @@ client->UpdateProvisionalAccountDisplayName("CoolPlayer123", [](discordpp::Clien
 
 ## Merging Provisional Accounts
 
-When a player wants to convert their provisional acount to a full Discord account.
+When a player wants to convert their provisional account to a full Discord account.
 
 ### Merging Provisional Accounts for Public Clients
 
@@ -349,6 +349,16 @@ def exchange_device_code_with_merge(device_code):
 }
 ```
 
+### Data Migration During Merging
+
+When a user merges their provisional account with a Discord account, the following data is automatically transferred:
+
+* **Friends**: All In-game and Discord friendships made through the provisional account
+* **Lobby Memberships**: Active and historical lobby participation
+* **❌ DM Messages**: Message history is not migrated during merging
+
+This migration ensures users don't lose their social connections built while using the provisional account.
+
 ---
 
 ## Unmerging Provisional Accounts
@@ -460,6 +470,19 @@ def unmerge_provisional_account(external_auth_token):
 If you have a server backend, you'll want to use the server-to-server unmerge endpoint rather than the SDK helper method to maintain better security and control over the unmerge process.
 :::
 
+### Relationship Transfer During Unmerging
+
+When a user unmerges their account, a new provisional account is created with a new user ID. The relationship transfer follows these rules:
+
+* **✅ In-game friends**: All copied to the new provisional account
+* **✅ Discord friends who use this application**: Copied to the provisional account
+* **❌ Discord friends who don't use this application**: Not transferred
+* **❌ DM message history**: Not moved to provisional accounts
+
+:::info
+Provisional accounts can have Discord friends, but can only message these friends when actively playing the game.
+:::
+
 ---
 
 ## Next Steps