3.x: Cleanup javadoc for parameter validation all around

[akarnokd]

• Add missing @throws documentation
• Fix wording and style of Flowable javadocs
• Fix documentation style of many files
• Improve the validator test to detect more patterns
• skip now rejects negative amounts with IllegalArgumentException

[codecov]

Codecov Report

Merging #6835 into 3.x will increase coverage by 0.05%.
The diff coverage is 97.1%.

@@             Coverage Diff              @@
##                3.x    #6835      +/-   ##
============================================
+ Coverage     98.23%   98.28%   +0.05%     
- Complexity     6379     6383       +4     
============================================
  Files           715      715              
  Lines         46502    46532      +30     
  Branches       6384     6388       +4     
============================================
+ Hits          45679    45734      +55     
+ Misses          304      283      -21     
+ Partials        519      515       -4

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	151 <0> (ø)	⬇️
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	576 <0> (+1)	⬆️
...o/reactivex/rxjava3/parallel/ParallelFlowable.java	100% <ø> (ø)	57 <0> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...reactivex/rxjava3/processors/UnicastProcessor.java	100% <100%> (ø)	66 <4> (ø)	⬇️
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <100%> (ø)	552 <8> (+1)	⬆️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <100%> (ø)	179 <3> (ø)	⬇️
.../reactivex/rxjava3/processors/ReplayProcessor.java	97.79% <100%> (+0.04%)	52 <4> (ø)	⬇️
...io/reactivex/rxjava3/subjects/BehaviorSubject.java	98.78% <100%> (-0.02%)	44 <3> (-1)
...activex/rxjava3/flowables/ConnectableFlowable.java	100% <100%> (ø)	12 <2> (ø)	⬇️
... and 32 more

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