Add marble diagram to the Single.filter method

[UMFsimke]

Hopefully this should tick off the filter operator in #5788

Please let me know if I need to change anything on the diagram itself. If not, please send me URL when you upload image within the project so I can change URL for the marble in the PR.

Although, I had a doubt and wanted to double check. Operator filter returns Maybe which essentially will call onSuccess or onComplete. By running jUnit tests, I could observe that this is a behavior but if I use TestObserver then both assertValue() and assertComplete() are invoked when there are elements that satisfy filter condition.

[codecov]

Codecov Report

Merging #6075 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6075      +/-   ##
============================================
- Coverage     98.25%   98.23%   -0.03%     
  Complexity     6191     6191              
============================================
  Files           666      666              
  Lines         44806    44806              
  Branches       6206     6206              
============================================
- Hits          44026    44017       -9     
- Misses          234      242       +8     
- Partials        546      547       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	92.3% <0%> (-6.84%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.99%)	2% <0%> (ø)
.../operators/mixed/FlowableConcatMapCompletable.java	97.43% <0%> (-2.57%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/parallel/ParallelReduceFull.java	93.06% <0%> (-1.99%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
... and 18 more

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[akarnokd]

https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.filter.png

TestObserver implements multiple consumer types and an onSuccess is translated into onNext+onComplete for an easier verification.

[UMFsimke]

PR is updated. Thanks for explanation 👍