2.x: Fix subscribeWith documentation examples

[dimsuz]

This PR improves a documentation of subscribeWith functions in Single, Maybe, Completable by making sure examples are compilable (by using classes which implement Disposable).

This should fix issue #5642 .

In that issue @akarnokd approved my idea to use DisposableMaybeObserver in the example, but as I looked in other classes, I noticed that Observable.subscribeWith() documentation uses a ResourceObserver (which implements Disposable), so I thought that this is the way to go in all other top-classes?

If you think that this is not the case, and instead all of them should be switched to use Disposable*Observer in their examples, let me know, I will do it in this way then.

I think that all those example should use similar classes.

[akarnokd]

ResourceXObserver is generally not necessary so DisposableXObserver should be in the docs.

[codecov]

Codecov Report

Merging #5647 into 2.x will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5647      +/-   ##
============================================
- Coverage     96.19%   96.16%   -0.04%     
- Complexity     5843     5847       +4     
============================================
  Files           634      634              
  Lines         41539    41539              
  Branches       5752     5752              
============================================
- Hits          39960    39945      -15     
- Misses          626      642      +16     
+ Partials        953      952       -1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	135 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	169 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	113 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	506 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	79.48% <0%> (-15.39%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <0%> (-2.69%)	7% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	93.04% <0%> (-2.14%)	5% <0%> (ø)
... and 26 more

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[dimsuz]

Got it, will update soon. Is it ok to force push into PR branches?

[akarnokd]

PR branches are in your own repo so yes.