2.x: Fix toFuture marbles and descriptions

[akarnokd]

This PR fixes the marble diagrams of Flowable.toFuture and Observable.toFuture to indicate:

• multi-valued sources signal IndexOutOfBoundsException
• Source has to be finite for the Future to terminate.

Resolves #6198

(also fixes the diagram for #5813).

[codecov]

Codecov Report

Merging #6200 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6200      +/-   ##
============================================
- Coverage     98.23%   98.22%   -0.02%     
+ Complexity     6202     6199       -3     
============================================
  Files           667      667              
  Lines         44888    44888              
  Branches       6218     6218              
============================================
- Hits          44095    44090       -5     
  Misses          253      253              
- Partials        540      545       +5

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	566 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	541 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-3.93%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	97.87% <0%> (-1.71%)	11% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-1.39%)	56% <0%> (-1%)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
... and 20 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e0532b7...d255138. Read the comment docs.