Clarify the specification for muxed accounts in CAP-64. (#1635)

core/cap-0064.md

@@ -136,6 +136,15 @@ A new type of of address credentials is introduced for `SorobanAuthorizationEntr
 
 The first version of the credentials will still be supported by the protocol. It can be considered to be semantically equivalent to `SOROBAN_CREDENTIALS_ADDRESS_V2` with `txMemo` set to `MEMO_NONE` (i.e. it may only pass authorization check when the transaction memo is `MEMO_NONE`).
 
+#### Muxed accounts restrictions
+
+In order to avoid confusion between memos and muxed source accounts, transactions that contain both of:
+
+- Muxed transaction source account and/or muxed operation source account
+- At least one Soroban authorization entry with credentials that are not set to `SOROBAN_CREDENTIALS_SOURCE_ACCOUNT`
+
+are considered invalid and thus they won't be ever included into ledger.
+
 ## Design Rationale
 
 The design is simply the minimal necessary set of changes that allows for signing transaction memos. We had to come up with the new XDR structures because the initial versions did not have any extension points. In order to simplify potential future changes, the extension point has been introduced as well.