3.x: Update observeOn docs with links and +backpressure explanation

[akarnokd]

Fixed a style mistake and wrong classes mentioned in the Flowable.observeOn docs.

In addition, I've added explanation about it not preserving the request pattern across.

[codecov]

Codecov Report

Merging #6750 into 3.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                3.x   #6750      +/-   ##
===========================================
- Coverage     98.16%   98.1%   -0.06%     
- Complexity     6188    6189       +1     
===========================================
  Files           677     677              
  Lines         44619   44619              
  Branches       6157    6157              
===========================================
- Hits          43800   43775      -25     
- Misses          292     301       +9     
- Partials        527     543      +16

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	559 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	88.02% <0%> (-5.64%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.7% <0%> (-3.65%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	95.14% <0%> (-2.92%)	6% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.11%)	3% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.61%)	5% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	95.9% <0%> (-0.82%)	2% <0%> (ø)
... and 11 more

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