consensus/doc: explain GetValueOut() precondition

`Consensus::CheckTxInputs` calls `tx.GetValueOut()` and assumes output-range checks already ran.
Document the mempool and block validation call paths where this is guaranteed.

Co-authored-by: Antoine Poinsot <darosior@protonmail.com>

src/consensus/tx_verify.cpp

@@ -188,6 +188,10 @@ bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state,
         }
     }
 
+    // `tx.GetValueOut()` won't throw in validation paths because output-range checks run first
+    // (`bad-txns-vout-negative`, `bad-txns-vout-toolarge`, `bad-txns-txouttotal-toolarge`):
+    // * `MemPoolAccept::PreChecks`: `CheckTransaction()` is called before this method;
+    // * `Chainstate::ConnectBlock`: `CheckTransaction()` is called via `CheckBlock()` before this method.
     const CAmount value_out = tx.GetValueOut();
     if (nValueIn < value_out) {
         return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-txns-in-belowout",