docs: clarify spl token account creation handling for exchange integrations (solana-labs#23288)

(cherry picked from commit 09d064c)

Co-authored-by: Trent Nelson <[email protected]>

Author: mergify[bot]

docs/src/integrations/exchange.md

@@ -721,6 +721,10 @@ must then be used to determine the effective balance change. These fields will
 identify the token mint and account owner (main wallet address) of the affected
 account.
 
+Note that if a receiving account is created during the transaction, it will have no
+`preTokenBalance` entry as there is no data to decode the state from.  In this
+case, the initial balance can be assumed to be zero.
+
 ### Withdrawing
 
 The withdrawal address a user provides must be the that of their SOL wallet.