2.x: Improve Javadocs of flatMapSingle and flatMapMaybe

[akarnokd]

Clarify that flatMapSingle and flatMapMaybe merge in no particular order and fix the copy-paste error from flatMapCompletable as they have values to merge, not just waiting.

Related: StackOverflow question

[codecov]

Codecov Report

Merging #5709 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #5709      +/-   ##
===========================================
- Coverage     96.3%   96.29%   -0.01%     
- Complexity    5821     5822       +1     
===========================================
  Files          634      634              
  Lines        41604    41604              
  Branches      5761     5761              
===========================================
- Hits         40068    40064       -4     
- Misses         608      612       +4     
  Partials       928      928

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	525 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	508 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	79.48% <0%> (-18.81%)	2% <0%> (ø)
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	92.47% <0%> (-2.16%)	9% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	98.3% <0%> (-1.7%)	11% <0%> (ø)
... and 20 more

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[artem-zinnatullin]

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