2.x: Maybe/Single Javadoc; annotation cleanup

[akarnokd]

The PR cleans up some wording around "Single source" and "Maybe source", plus removes some unnecessary annotations.

[codecov]

Codecov Report

Merging #5977 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5977      +/-   ##
============================================
- Coverage     98.26%   98.24%   -0.02%     
- Complexity     6051     6053       +2     
============================================
  Files           656      656              
  Lines         44077    44077              
  Branches       6118     6118              
============================================
- Hits          43312    43304       -8     
- Misses          228      232       +4     
- Partials        537      541       +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	91.79% <0%> (-2.99%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-1.31%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-0.97%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
... and 19 more

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