2.x: cleanup for text and javadoc 04/15

[akarnokd]

This PR

• fixes a couple of documentation and message copy-paste errors
• fixed FlowableFlattenIterable not checking for null returned by Iterator::next()
• replace publisher with the appropriate name

[codecov]

Codecov Report

Merging #5286 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5286      +/-   ##
============================================
- Coverage     96.13%   96.11%   -0.02%     
- Complexity     5748     5755       +7     
============================================
  Files           628      628              
  Lines         41077    41077              
  Branches       5699     5699              
============================================
- Hits          39488    39481       -7     
- Misses          621      626       +5     
- Partials        968      970       +2

Impacted Files	Coverage Δ	Complexity Δ
...o/reactivex/observables/ConnectableObservable.java	100% <ø> (ø)	7 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	168 <0> (ø)	⬇️
.../java/io/reactivex/subjects/SerializedSubject.java	95.78% <ø> (ø)	27 <0> (ø)	⬇️
...rators/observable/ObservableBlockingSubscribe.java	96.96% <ø> (ø)	9 <0> (ø)	⬇️
...al/operators/flowable/FlowableFlattenIterable.java	97.24% <100%> (ø)	4 <0> (ø)	⬇️
...s/observable/ObservableBufferBoundarySupplier.java	90.67% <100%> (ø)	2 <0> (ø)	⬇️
...internal/operators/observable/ObservableDefer.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...perators/observable/ObservableMapNotification.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/observable/ObservableDebounce.java	94.87% <100%> (ø)	2 <0> (ø)	⬇️
...in/java/io/reactivex/subjects/BehaviorSubject.java	84.89% <0%> (-5.73%)	56% <0%> (ø)
... and 42 more

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