docs: signing what is sent, and which --sign-every to choose

api.md's Signed changes says what sent, all and none do, what each
costs, and which to choose: sent for nearly every swarm, all where each
change should carry its own proof, none to save signatures where only
protocol data and listed paths need them; and that signatures travel
with their changes, a message carrying none taking none. access.md no
longer says signatures come on their own within a second.

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