2.x: fix CallbackCompletableObserver calling onError, ResourceX wording

[akarnokd]

This PR contains the fix for incorrect call of the onError handler within CallbackCompletableObserver.onComplete() reported in 5099 as well as wording fixes to the ResourceX javadoc.

[codecov]

Codecov Report

Merging #5240 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5240      +/-   ##
============================================
+ Coverage     96.12%   96.15%   +0.02%     
- Complexity     5757     5758       +1     
============================================
  Files           628      628              
  Lines         41074    41073       -1     
  Branches       5697     5697              
============================================
+ Hits          39484    39495      +11     
+ Misses          630      616      -14     
- Partials        960      962       +2

Impacted Files	Coverage Δ	Complexity Δ
.../io/reactivex/observers/ResourceMaybeObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
...io/reactivex/observers/ResourceSingleObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
...activex/observers/ResourceCompletableObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
...ava/io/reactivex/observers/DisposableObserver.java	100% <ø> (ø)	7 <0> (ø)	⬇️
...nternal/observers/CallbackCompletableObserver.java	100% <100%> (ø)	9 <0> (ø)	⬇️
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-8.2%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	91.5% <0%> (-5.23%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-3.51%)	10% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...activex/internal/subscribers/FutureSubscriber.java	96.82% <0%> (-3.18%)	32% <0%> (-1%)
... and 42 more

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