[docs] Clarify that authwits only work in single player mode

[critesjosh]

Authwits will only work in "single player multi contract" mode i.e. I won't be able to send you authwit to transfer on my behalf. But I can still do private/public authwits to defi contracts etc. (i.e. I can't authorize another human/account contract but can do so for a normal contract).