docs: note that Execute.amount is u64::MAX for confidential transfers

[pwsaragossy]

The doc on TransferHookInstruction::Execute.amount reads "Amount of tokens to transfer". On the confidential-transfer path that is not accurate: Token-2022 passes u64::MAX, because the amount is encrypted and unavailable to the program.

https://github.com/solana-program/token-2022/blob/6d87d47d6bbd/program/src/extension/confidential_transfer/processor.rs#L826-L836

The convention is deliberate and there is a comment at the call site. Nothing on the receiving side says so — u64::MAX does not appear anywhere in this repository. A hook implementing amount-based policy reads this field's documentation and has no reason to expect a sentinel.

The consequence depends on how the policy is written:

• a per-transfer cap (amount > limit) rejects every confidential transfer, at any size
• an accumulator (total + amount) overflows; with overflow-checks off, Cargo's release default, it wraps and the check silently passes

Documentation only, no behavior change.

Reproduced against a pinned commit using upstream's own tests, unmodified — the only change is an instrumented hook that reports the amount it was handed: https://github.com/pwsaragossy/t22-conformance

[joncinque]

Thanks for your contribution! Just a little bit to trim, then it's good to go on my side

[joncinque]

Applied suggestion, landing