3.x: Javadocs: indicate takeUntil stops on completion of other

[akarnokd]

Update the Javadocs of the takeUntil() operator to explicitly mention it stops if the other source just completes without an item.

Images: #7339 (comment)

Resolves #7339

[codecov]

Codecov Report

Merging #7341 (6e7de23) into 3.x (54114b9) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7341      +/-   ##
============================================
- Coverage     99.52%   99.50%   -0.02%     
  Complexity     6780     6780              
============================================
  Files           751      751              
  Lines         47482    47482              
  Branches       6378     6378              
============================================
- Hits          47255    47247       -8     
- Misses          104      106       +2     
- Partials        123      129       +6     

Impacted Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-3.97%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	93.65% <0.00%> (-3.18%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	95.07% <0.00%> (-2.12%)	⬇️
...o/reactivex/rxjava3/processors/AsyncProcessor.java	97.91% <0.00%> (-2.09%)	⬇️
...vable/ObservableFlatMapCompletableCompletable.java	98.48% <0.00%> (-1.52%)	⬇️
...ators/observable/ObservableFlatMapCompletable.java	98.52% <0.00%> (-1.48%)	⬇️
...internal/operators/flowable/FlowableSwitchMap.java	98.58% <0.00%> (-1.42%)	⬇️
...rnal/operators/flowable/FlowableSequenceEqual.java	98.78% <0.00%> (-1.22%)	⬇️
... and 13 more

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