2.x: Add missing Completable marbles (+17, 07/18a)

[akarnokd]

This PR adds marble diagrams to various Completable operators:

wrap

using

ambWith

andThen

as, to

compose

cache

blockingAwait

blockingGet

[codecov]

Codecov Report

Merging #6090 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6090      +/-   ##
============================================
+ Coverage     98.25%   98.25%   +<.01%     
+ Complexity     6197     6195       -2     
============================================
  Files           667      667              
  Lines         44858    44858              
  Branches       6211     6211              
============================================
+ Hits          44074    44076       +2     
+ Misses          243      237       -6     
- Partials        541      545       +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	117 <0> (ø)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	97.61% <0%> (-1.2%)	2% <0%> (ø)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
... and 22 more

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