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

[JosemyDuarte]

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

Resolves #6697

[codecov]

Codecov Report

Merging #6700 into 3.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6700      +/-   ##
============================================
+ Coverage     98.12%   98.14%   +0.01%     
+ Complexity     6189     6188       -1     
============================================
  Files           677      677              
  Lines         44657    44657              
  Branches       6170     6170              
============================================
+ Hits          43821    43827       +6     
+ Misses          300      294       -6     
  Partials        536      536

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	559 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	537 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.1% <0%> (-3.13%)	2% <0%> (ø)
...activex/rxjava3/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...rxjava3/internal/schedulers/ExecutorScheduler.java	96% <0%> (-1.5%)	10% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.09% <0%> (-0.91%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	97.08% <0%> (-0.65%)	6% <0%> (ø)
... and 13 more

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[akarnokd]

Please put the text after the image and the see as last.

[akarnokd]

Thanks! Much appreciated!

Would you like to backport the changes to the 2.x branch?