2.x: additional warnings for fromPublisher()

[akarnokd]

This PR adds JavaDoc warnings for fromPublisher as people still try to implement a source through it and fail to follow the specification like this:

// DON'T DO THIS!
Flowable.fromPublisher((Subscriber<Integer> s) -> s.onNext(1));

Even though Publisher is a single abstract method (SAM) type, it is almost never reasonable to write a source with it that doesn't require a per-subscriber state. The example above fails to setup backpressure properly and it didn't call s.onSubscribe() for that, causing NullPointerException whenever the Subscription would be needed (i.e., for cancellation and requesting) by the downstream operator.

Maybe doesn't have fromPublisher.

[codecov]

Codecov Report

Merging #5640 into 2.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5640      +/-   ##
============================================
- Coverage     96.27%   96.22%   -0.06%     
+ Complexity     5852     5844       -8     
============================================
  Files           634      634              
  Lines         41539    41539              
  Branches       5752     5752              
============================================
- Hits          39993    39972      -21     
- Misses          611      623      +12     
- Partials        935      944       +9

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	523 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	113 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	135 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	88.49% <0%> (-4.87%)	62% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	61.7% <0%> (-2.84%)	33% <0%> (-2%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
... and 29 more

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[vanniktech]

maybe add which is here? state management which is not achievable with a stateless lambda.

[akarnokd]

Or that is. Grammarly doesn't complain about the PR's text.

[vanniktech]

That is would also work.