Document Vesting curves invariants

[ericnordelo]

The Vesting component allows developers to implement custom vesting schedules by defining their own logic through the VestingScheduleTrait. While this provides flexibility, it does not prevent developers from creating schedules that could interact poorly with permissionless deposits. For example, if a vesting curve fails to satisfy the property vested_t(allocation) + x < vested_t(allocation + x), a beneficiary could accelerate their vesting by depositing additional funds.