3.x: Improve Javadocs of Single

[akarnokd]

• Use {@link}s and {@code} tags
• Fix some wording and copy-paste mistakes
• Add @throws about argument validation throwing NullPointerExceptions and IllegalArgumentExceptions

[codecov]

Codecov Report

Merging #6812 into 3.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6812      +/-   ##
============================================
+ Coverage     98.24%   98.27%   +0.03%     
- Complexity     6385     6387       +2     
============================================
  Files           715      715              
  Lines         46498    46498              
  Branches       6371     6371              
============================================
+ Hits          45680    45694      +14     
+ Misses          295      287       -8     
+ Partials        523      517       -6

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	153 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-3.97%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.18% <0%> (-1.17%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	98.02% <0%> (-0.99%)	49% <0%> (-1%)
...a3/internal/operators/flowable/FlowableReplay.java	91.99% <0%> (-0.83%)	19% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-0.54%)	5% <0%> (ø)
.../reactivex/rxjava3/processors/ReplayProcessor.java	97.34% <0%> (-0.41%)	52% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	95.59% <0%> (ø)	3% <0%> (ø)	⬇️
... and 11 more

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