3.x: Add last missing throws tag to JavaDocs

[akarnokd]

Added the last missing @throws tag to methods and one missing @NonNull annotation.

I included a small program that will list JavaDocs without @throws for future use.

Resolves #6829

[codecov]

Codecov Report

Merging #6860 into 3.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6860      +/-   ##
============================================
+ Coverage     98.24%   98.28%   +0.03%     
- Complexity     6379     6381       +2     
============================================
  Files           715      715              
  Lines         46524    46524              
  Branches       6387     6387              
============================================
+ Hits          45709    45727      +18     
+ Misses          294      286       -8     
+ Partials        521      511      -10

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> (ø)	⬇️
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	93.57% <0%> (-4.59%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.55% <0%> (-2.47%)	50% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.27% <0%> (-2.09%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-1.59%)	2% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-0.84%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	95.9% <0%> (-0.82%)	2% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	96.22% <0%> (+0.31%)	3% <0%> (ø)	⬇️
... and 12 more

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