2.x: marble additions and updates (12/11)

[akarnokd]

This PR adds or updates mable diagrams of Observable:

• doOnTerminate
• elementAt
• elementAt with default
• elementAtOrError
• firstOrError
• flatMap with error delayed
• flatMap with maximum concurrency

Related: #5319 (comment)

[codecov]

Codecov Report

Merging #5759 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5759      +/-   ##
============================================
+ Coverage     96.24%   96.26%   +0.01%     
- Complexity     5838     5840       +2     
============================================
  Files           634      634              
  Lines         41653    41653              
  Branches       5769     5769              
============================================
+ Hits          40090    40097       +7     
+ Misses          624      617       -7     
  Partials        939      939

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	509 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-5.23%)	2% <0%> (ø)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...java/io/reactivex/processors/UnicastProcessor.java	94.73% <0%> (-1.17%)	63% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.13% <0%> (-0.89%)	61% <0%> (-1%)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	88.23% <0%> (-0.66%)	2% <0%> (ø)
... and 22 more

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