Document the probability of finality in cases where F3 is slow in progressing

[masih]

Concretely document the likelihood of F3 leading the chain away into a fork relative to the time it takes for the latest F3 instance to finalize. The context for this ask is to understand what is considered a "safe" distance from head in a case where F3 finalized tipset is far behind.

For reference the Eth Filecoin APIs use the distance of 30 based on FRC-0089 finality calculator.

Explicit ask from ecosystem:

provide a summary for the F3 FAQ on probability of finality after 10 blocks, 30 blocks, etc. or whatever values make sense? There also needs to be an upper limit to confirmations.

Relates to:

• The F3 <-> EC fork issue #718

[BigLep]

For more context, this came out of the discussion for what value of "safe" to use in filecoin-project/lotus#13034 . In that PR, we settled on 200.

In filecoin-project/lotus#13051 the user-facing docs and constant were updated to link to this issue.