3.x: Improve Javadocs style of Schedulers

[akarnokd]

Add the proper links and code blocks to improve style to io.reactivex.rxjava3.schedulers.Schedulers.

[codecov]

Codecov Report

Merging #7168 (2396f77) into 3.x (80a4842) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## 3.x #7168 +/- ## ============================================ - Coverage 99.55% 99.53% -0.03%  + Complexity 6709 6708 -1  ============================================ Files 745 745 Lines 47332 47345 +13 Branches 6376 6380 +4 ============================================ Hits 47123 47123 - Misses 93 100 +7  - Partials 116 122 +6 

Impacted Files	Coverage Δ	Complexity Δ
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100.00% <ø> (ø)	12.00 <0.00> (ø)
...ors/flowable/FlowableWindowSubscribeIntercept.java	87.50% <0.00%> (-12.50%)	4.00% <0.00%> (-1.00%)
...l/operators/observable/ObservableFlatMapMaybe.java	94.36% <0.00%> (-2.82%)	2.00% <0.00%> (ø%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	50.00% <0.00%> (ø%)
...ternal/operators/observable/ObservablePublish.java	97.29% <0.00%> (-1.81%)	16.00% <0.00%> (-1.00%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.81% <0.00%> (-1.56%)	2.00% <0.00%> (ø%)
...java3/internal/operators/flowable/FlowableZip.java	98.97% <0.00%> (-1.03%)	6.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-0.80%)	2.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableGroupBy.java	99.45% <0.00%> (-0.55%)	2.00% <0.00%> (ø%)
.../reactivex/rxjava3/observers/BaseTestConsumer.java	99.46% <0.00%> (-0.54%)	74.00% <0.00%> (-1.00%)
... and 7 more

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