2.x: Add Completable marble diagrams (07/17a)

[akarnokd]

This PR adds the following marble diagrams to Completable operators (#5789):

amb, ambArray

complete

concatArray, concat

defer

error

[codecov]

Codecov Report

Merging #6083 into 2.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6083      +/-   ##
============================================
+ Coverage     98.25%   98.31%   +0.05%     
+ Complexity     6195     6194       -1     
============================================
  Files           667      667              
  Lines         44856    44856              
  Branches       6211     6211              
============================================
+ Hits          44074    44098      +24     
+ Misses          237      228       -9     
+ Partials        545      530      -15

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	116 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-6.72%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	93.08% <0%> (-3.2%)	3% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowableGroupBy.java	94.67% <0%> (-0.85%)	3% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	96.61% <0%> (-0.49%)	8% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.69% <0%> (-0.42%)	50% <0%> (ø)
... and 20 more

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