3.x: Simplify @return sentences in Flowable JavaDocs

[akarnokd]

Simplify the wording of @return in JavaDocs.

Resolves #6834

[codecov]

Codecov Report

Merging #6847 into 3.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6847      +/-   ##
============================================
+ Coverage     98.25%   98.26%   +0.01%     
+ Complexity     6380     6377       -3     
============================================
  Files           715      715              
  Lines         46522    46522              
  Branches       6387     6387              
============================================
+ Hits          45711    45716       +5     
- Misses          287      291       +4     
+ Partials        524      515       -9

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	576 <0> (ø)	⬇️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	178 <0> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	151 <0> (ø)	⬇️
...o/reactivex/rxjava3/parallel/ParallelFlowable.java	100% <ø> (ø)	57 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	551 <0> (ø)	⬇️
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	89.43% <0%> (-2.12%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
... and 21 more

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