CH03::malleability: correct statement about malleability after confirmation

Txids can be mutated after confirmation, it's just rare.  Update text to
state this.

ch03.asciidoc

@@ -787,12 +787,14 @@ a0100000000ffffffff02204e0000000000002251203b41daba4c9ace578369740f15e5ec880c&#x
 
 [TIP]
 ====
-((("transaction IDs (txd)")))((("malleability")))A transaction ID is not
-authoritative until a transaction has been confirmed. Absence of a
-transaction hash in the blockchain does not mean the transaction was not
-processed. This is known as "transaction malleability," because
-transaction hashes can be modified prior to confirmation in a block.
-After confirmation, the +txid+ is immutable and authoritative.
+((("transaction IDs (txd)")))((("malleability")))A transaction ID (txid)
+is not authoritative. Absence of a txid in the blockchain does not mean
+the transaction was not processed. This is known as "transaction
+malleability," because transactions can be modified prior to
+confirmation in a block, changing their txids.  After a transaction is
+included in a block, its txid cannot change unless there is a blockchain
+reorganization where that block is removed from the best blockchain.
+Reorganizations are rare after a transaction has several confirmations.
 ====
 
 The command +getrawtransaction+ returns a serialized transaction in