fix issues in streams pointed out by Jason

[rkuhn]

See @retronym’s comments on 248c0a3.

[patriknw]

In summary:

• change that Flow.apply thunk to accept a closure that returns Some/None instead
• document and test the behaviour for negative n in take and drop
• documenting the behaviour with exceptions from operators
• Use of uncheckedVariance ought to be justified with a comment. ("It is safe because ... / We can't use bounded method type params instead because ...")
• Scaladoc should either link or backtick things like FlowMaterializer
• documentation or more thought around thread local based withCtx, it relies on that the sub-flow is always created on the calling thread

[ktoso]

Notes to the todo list in patrik's comment:

• withCtx has been removed because we changed who creates the actors
• uncheckedVariance was addressed in +str #15408 Replace uncheckedVariance annotations in Flow/Duct #15643
• exceptions in operators will be addressed during implementing supervision in streams, the issue is: implement supervision scheme for streams #15710