2.x: More marbles 01/08-a

[akarnokd]

This PR adds 5 marble diagrams to Observable:

• flatMapCompletable (2 overloads)
• forEach
• forEachWhile
• hide

Images in #5319 (comment)

[codecov]

Codecov Report

Merging #5795 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5795      +/-   ##
============================================
+ Coverage     96.29%   96.32%   +0.03%     
+ Complexity     5808     5807       -1     
============================================
  Files           634      634              
  Lines         41631    41631              
  Branches       5776     5776              
============================================
+ Hits          40089    40102      +13     
+ Misses          602      590      -12     
+ Partials        940      939       -1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	509 <0> (ø)	⬇️
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...io/reactivex/subscribers/SerializedSubscriber.java	94.31% <0%> (-1.14%)	25% <0%> (-1%)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.06% <0%> (-0.9%)	59% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	96.58% <0%> (-0.86%)	2% <0%> (ø)
...tors/observable/ObservableSequenceEqualSingle.java	98.29% <0%> (-0.86%)	3% <0%> (ø)
...ain/java/io/reactivex/subjects/UnicastSubject.java	97.46% <0%> (-0.64%)	60% <0%> (-1%)
...nternal/operators/observable/ObservableReplay.java	98.05% <0%> (-0.56%)	23% <0%> (ø)
... and 18 more

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