Improve documentation for `--fork-retry-backoff`

[PaulRBerg]

https://github.com/foundry-rs/book/pull/637 added documentation for --fork-retry-backoff, but it is still unclear (to me) what this setting does.

Initial retry backoff on encountering errors.

What does "backoff" mean? Is it a number? An amount of time? A string? Is it enabled by default? If yes, what is the default value?

Also: is this the same thing as what Alchemy means by retries?

[PaulRBerg]

Related: https://github.com/foundry-rs/foundry/issues/4958