Update docs with new `Arbitrary` API based on `ArbMap` about fscheck HOT 6 OPEN

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Per #638, I have a:

module ArbMap =
    let defGen<'t> = ArbMap.defaults |> ArbMap.generate<'t>

That I sprinkle around

The changes are listed in https://github.com/fscheck/FsCheck/blob/master/FsCheck%20Release%20Notes.md

🤔 (I found it by searching for generate) Perhaps there could be an explicit long form summary of moved methods in there so people can search for the string Arb.generate etc ? Or maybe if the changelog was formatted a la KeepAChangeLog e.g. https://github.com/jet/equinox/blob/master/CHANGELOG.md#changed, then you could look up Arb in the changes ?

from fscheck.

What would help me - as someone new to the library in general - is a very basic example of how to use the built in generators to build your own generators.

Per #638, I have a:

module ArbMap =
    let defGen<'t> = ArbMap.defaults |> ArbMap.generate<'t>

That I sprinkle around

This is equivalent to the old Arb.generate?

from fscheck.

Yes - it's equivalent. See this PR for some code that uses it https://github.com/jet/equinox/pull/412/files

In general I'd recommend downloading the FsCheck solution and doing a text search within in - there's some examples and you also get to see the relationships. The changelog linked above actually does detail the transitions quite well if you read it carefully, but I've been in your shoes wondering where all the cheese has gone and when the yaks are going to part ways and let you move on, so I can definitely empathise...

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