update f3 FIP-0086 with deltaBackOffMax

[hanabi1224]

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No. This PR admends FIP-0086 to address filecoin-project/go-f3#1056.

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[rvagg]

from an editorial perspective this is fine, but it'll need technical acceptance and go-f3 updates to match, so we'll wait for @Kubuxu's review here

[BigLep]

We're going to discuss risks / next steps of this work in filecoin-project/go-f3#1056 . I'll mark this as draft until this gets hashed out by implementers.