doc: Add note to getblockfrompeer on missing undo data

src/rpc/blockchain.cpp

@@ -429,6 +429,7 @@ static RPCHelpMan getblockfrompeer()
         "getblockfrompeer",
         "Attempt to fetch block from a given peer.\n\n"
         "We must have the header for this block, e.g. using submitheader.\n"
+        "The block will not have any undo data which can limit the usage of the block data in a context where the undo data is needed.\n"
         "Subsequent calls for the same block may cause the response from the previous peer to be ignored.\n"
         "Peers generally ignore requests for a stale block that they never fully verified, or one that is more than a month old.\n"
         "When a peer does not respond with a block, we will disconnect.\n"