Merge bitcoin/bitcoin#32129: doc: Update comments for AreInputsStandard to match code

52ede28 doc: Update comments for AreInputsStandard to match code (Anthony Towns)

Pull request description:

  The comment about extra data stuffed in scriptSigs was introduced in #4365 which introduced `ScriptSigArgsExpected()`, and became incorrect after #7387 / #7453 (checks are now performed by `SCRIPT_VERIFY_CLEANSTACK` during script validation and `IsPushOnly()` in `IsStandardTx()`). Drops the details on what a p2sh with many checksigs would look like, which was already done in #4365, but only for main.cpp not the duplicated comment in main.h, which was merged into policy/policy.cpp in #6335 and later moved to the right place in #10682.

ACKs for top commit:
  instagibbs:
    ACK 52ede28
  darosior:
    ACK 52ede28

Tree-SHA512: 5ee9a775c81d4c23aca2f8f938ab8bfa7605af489ddb78788613195be8744c7fb7a37bae271093f67f572577452651d4958706b55346e99cf8d32ac0fc34df03

Author: fanquake

src/policy/policy.cpp

@@ -169,20 +169,18 @@ bool IsStandardTx(const CTransaction& tx, const std::optional<unsigned>& max_dat
 }
 
 /**
- * Check transaction inputs to mitigate two
- * potential denial-of-service attacks:
+ * Check transaction inputs.
  *
- * 1. scriptSigs with extra data stuffed into them,
- *    not consumed by scriptPubKey (or P2SH script)
- * 2. P2SH scripts with a crazy number of expensive
- *    CHECKSIG/CHECKMULTISIG operations
- *
- * Why bother? To avoid denial-of-service attacks; an attacker
- * can submit a standard HASH... OP_EQUAL transaction,
- * which will get accepted into blocks. The redemption
- * script can be anything; an attacker could use a very
- * expensive-to-check-upon-redemption script like:
- *   DUP CHECKSIG DROP ... repeated 100 times... OP_1
+ * This does three things:
+ *  * Prevents mempool acceptance of spends of future
+ *    segwit versions we don't know how to validate
+ *  * Mitigates a potential denial-of-service attack with
+ *    P2SH scripts with a crazy number of expensive
+ *    CHECKSIG/CHECKMULTISIG operations.
+ *  * Prevents spends of unknown/irregular scriptPubKeys,
+ *    which mitigates potential denial-of-service attacks
+ *    involving expensive scripts and helps reserve them
+ *    as potential new upgrade hooks.
  *
  * Note that only the non-witness portion of the transaction is checked here.
  */