3.x: observeOn and Schedulers.from eagerness javadoc updates

[akarnokd]

• Add missing explanation to observeOn(Scheduler)
• Remove <p>s that create javadoc empty-tag warnings.
• Update @see to point to delays with delayError parameter.
• Explain that Schedulers.from is eager too.

Related #6700

[codecov]

Codecov Report

Merging #6723 into 3.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6723      +/-   ##
============================================
- Coverage     98.13%   98.11%   -0.02%     
- Complexity     6190     6191       +1     
============================================
  Files           677      677              
  Lines         44663    44663              
  Branches       6171     6171              
============================================
- Hits          43829    43822       -7     
- Misses          296      302       +6     
- Partials        538      539       +1

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	559 <0> (ø)	⬇️
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100% <ø> (ø)	11 <0> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	537 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	92.3% <0%> (-5.13%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...java3/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-1.58%)	3% <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)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-0.84%)	2% <0%> (ø)
... and 16 more

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