Update PSBT invariants

Author: NicolasDorier

bip-0078.mediawiki

@@ -109,14 +109,15 @@ The original PSBT MAY:
 
 The payjoin proposal MUST:
 * Use all the inputs from the original PSBT.
-* Only finalize the inputs added by the receiver.
-* Have all the <code>witnessUTXO</code> or <code>nonWitnessUTXO</code> information filled in.
-* Keep all the outputs unrelated to the payment.
+* Use all the outputs which do not belongs to the receiver from the original PSBT.
+* Only finalize the inputs added by the receiver. (Referred later as <code>additional inputs</code>)
+* Only fill the <code>witnessUTXO</code> or <code>nonWitnessUTXO</code> for the additional inputs.
 
 The payjoin proposal MAY:
-* Add inputs.
-* Add outputs.
-* Modify payment's output.
+* Add, remove or modify the outputs belonging to the receiver.
+
+The payjoin proposal MUST NOT:
+* Shuffle the order of inputs or outputs, the additional outputs or additional inputs must be inserted at a random index.
 
 ===Receiver's well known errors===