refactor: improves Creating-Observables wiki doc

[worstkiller]

This PR improves the documentation for creating observables
https://github.com/ReactiveX/RxJava/blob/2.x/docs/Creating-Observables.md
suggestions from @lorenzpahl also included.
Closes: #6435

[codecov]

Codecov Report

Merging #6436 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6436      +/-   ##
============================================
+ Coverage     98.29%   98.31%   +0.01%     
+ Complexity     6297     6296       -1     
============================================
  Files           675      675              
  Lines         45172    45172              
  Branches       6246     6246              
============================================
+ Hits          44404    44410       +6     
- Misses          235      236       +1     
+ Partials        533      526       -7

Impacted Files	Coverage Δ	Complexity Δ
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-4.17%)	55% <0%> (-3%)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-2.18%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...ain/java/io/reactivex/subjects/UnicastSubject.java	99.36% <0%> (-0.64%)	64% <0%> (-1%)
.../operators/observable/ObservableCombineLatest.java	96.31% <0%> (-0.62%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.75% <0%> (-0.49%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	89.32% <0%> (-0.33%)	3% <0%> (ø)
... and 15 more

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