Docs: explain OUTPACE unidirectionality

[Ivshti]

explain:

• why sequence is not needed (cause we can just track how much it was withdrawn so far)
• why a challenge period is not needed (same reason)
• why the channel deposit has to be provided by the same party that is the leading validator (to align incentives and make validation easier)
• why using an old state is harmless, and why we still need 2/2 multisig (to ensure that the leader can't go back on their payment)