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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1e4dbcf...939b3fc. Read the comment docs.