2.x: improve the documentation of Schedulers utility class.

[akarnokd]

This PR improves the javadoc of the io.reactivex.schedulers.Schedulers utility class plus adds a missing SchedulerSupport constant for the single() scheduler.

Let me know if something needs further explanation (or some new case needs explanation).

Related: #5145.

[codecov]

Codecov Report

Merging #5223 into 2.x will decrease coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x    #5223     +/-   ##
===========================================
- Coverage     96.01%   95.91%   -0.1%     
+ Complexity     5747     5740      -7     
===========================================
  Files           628      628             
  Lines         41085    41085             
  Branches       5698     5698             
===========================================
- Hits          39446    39406     -40     
- Misses          657      679     +22     
- Partials        982     1000     +18

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/schedulers/Schedulers.java	100% <ø> (ø)	10 <0> (ø)	⬇️
...a/io/reactivex/processors/SerializedProcessor.java	91.48% <0%> (-6.39%)	27% <0%> (-1%)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	93.87% <0%> (-6.13%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	89.55% <0%> (-5.98%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	92.45% <0%> (-5.67%)	23% <0%> (-2%)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	89.83% <0%> (-3.21%)	5% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	96.42% <0%> (-2.68%)	2% <0%> (ø)
... and 36 more

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