Explicit concurrency option for `Stream.flatMap`

[fiws]

What is the problem this feature would solve?

I debugged my code for a long time not noticing that Stream.flatMap takes a second argument where one can configure the concurrency.

What is the feature you are proposing to solve the problem?

Make the concurrency option NOT optional, like with Stream.mergeAll.

What alternatives have you considered?

• Improve docs: While the docs on the website are already doing a very good job in explaining this, the tsdoc could also be improved to mention the concurrency option.
• Get good (skill issue maybe? close the issue then 😅 )