2.x: Update javadoc for observeOn to mention its eagerness

[akarnokd]

Backport of #6700

Update javadoc to reflect the eagerness property of observeOn in Flowable and Observable classes.

Related #6697

[codecov]

Codecov Report

Merging #6722 into 2.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6722      +/-   ##
============================================
- Coverage     98.26%   98.21%   -0.05%     
- Complexity     6346     6349       +3     
============================================
  Files           677      677              
  Lines         45527    45527              
  Branches       6333     6333              
============================================
- Hits          44737    44715      -22     
- Misses          246      256      +10     
- Partials        544      556      +12

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	567 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	542 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.98%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-1.97%)	2% <0%> (ø)
...nal/operators/observable/ObservablePublishAlt.java	94.39% <0%> (-1.87%)	15% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.27% <0%> (-1.45%)	2% <0%> (ø)
... and 20 more

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