3.x: Javadocs fix some wording in Schedulers.java

[akarnokd]

Fix the sentence of the fair parameter and add a note about overriding the initial schedulers before class initialization.

[codecov]

Codecov Report

Merging #7301 (aa51fb1) into 3.x (a463d24) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                3.x    #7301   +/-   ##
=========================================
  Coverage     99.52%   99.52%           
  Complexity     6783     6783           
=========================================
  Files           751      751           
  Lines         47489    47489           
  Branches       6378     6378           
=========================================
  Hits          47265    47265           
+ Misses          102      100    -2     
- Partials        122      124    +2     

Impacted Files	Coverage Δ
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100.00% <ø> (ø)
...erators/completable/CompletableConcatIterable.java	95.91% <0.00%> (-4.09%)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	⬇️
...o/reactivex/rxjava3/processors/AsyncProcessor.java	97.91% <0.00%> (-2.09%)	⬇️
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	96.33% <0.00%> (-1.84%)	⬇️
...nternal/operators/parallel/ParallelSortedJoin.java	98.49% <0.00%> (-1.51%)	⬇️
...nternal/operators/observable/ObservableCreate.java	99.14% <0.00%> (-0.86%)	⬇️
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	99.16% <0.00%> (-0.42%)	⬇️
...ternal/operators/observable/ObservableFlatMap.java	97.87% <0.00%> (-0.36%)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	96.47% <0.00%> (ø)
... and 7 more

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