2.x: update javadoc of flatMap() overloads

[akarnokd]

This PR rewords the Javadoc of Flowable.flatMap() overloads to correctly express their behavior regarding backpressure towards their upstream. In 2.x, only merge() operators are unbounded-in by default, flatMaps are by default bounded by bufferSize() or the maxConcurrency parameter.

In addition, one of the flatMap overload still had resultSelector instead of the common combiner parameter name. As far as I know, there are no language-specific libraries targeting 2.x yet where in the language the actual parameter names are also part of the signature and changing them counts as an incompatible change.

Related #5126.

[codecov]

Codecov Report

Merging #5127 into 2.x will decrease coverage by -0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5127      +/-   ##
============================================
- Coverage     95.93%   95.87%   -0.07%     
+ Complexity     5605     5601       -4     
============================================
  Files           620      620              
  Lines         39810    39810              
  Branches       5597     5597              
============================================
- Hits          38191    38167      -24     
- Misses          637      661      +24     
  Partials        982      982

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	522 <1> (ø)	❌
...rnal/subscriptions/ArrayCompositeSubscription.java	93.33% <ø> (-6.67%)	17% <ø> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	88.52% <ø> (-6.56%)	2% <ø> (ø)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <ø> (-6.13%)	2% <ø> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.93% <ø> (-5.7%)	55% <ø> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	86.44% <ø> (-5.15%)	55% <ø> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <ø> (-5.13%)	14% <ø> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <ø> (-4.62%)	27% <ø> (-1%)
.../main/java/io/reactivex/subjects/MaybeSubject.java	95.65% <ø> (-4.35%)	46% <ø> (-2%)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <ø> (-3.58%)	2% <ø> (ø)
... and 38 more

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