2.x: Update Maybe doOn{Success,Error,Complete} JavaDoc

[akarnokd]

This PR updates the JavaDoc of the Maybe.doOnSuccess, Maybe.doOnError and Maybe.doOnComplete marble diagrams and adds an explicit sentence about mutually exclusive events.

[codecov]

Codecov Report

Merging #5785 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5785      +/-   ##
============================================
- Coverage     96.24%   96.22%   -0.02%     
- Complexity     5808     5812       +4     
============================================
  Files           634      634              
  Lines         41607    41607              
  Branches       5770     5770              
============================================
- Hits          40044    40036       -8     
- Misses          617      634      +17     
+ Partials        946      937       -9

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	170 <0> (ø)	⬇️
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	90.13% <0%> (-5.83%)	59% <0%> (-1%)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.24% <0%> (-5.83%)	54% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-3.27%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <0%> (-2.78%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.24%)	2% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
... and 25 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ba1f40f...7fc3cc4. Read the comment docs.