Initial clean up for Combining Observables docs

[ahmedre]

This patch updates the style of the combining observables documentation to be similar to that of #6131 and adds examples for most of the operators therein. Refs #6132.

What's left for combining observables:

• documentation for join, groupJoin, and rxjava-joins
• add documentation for concat, concatEager, and withLatestFrom

[ahmedre]

i was unsure whether to go with an example like this one or something more abstract but potentially more "useful" like:

Observable<NewsItems> latestNews = getLatestNews();
Observable<Weather> latestWeather = getLatestWeather();
Observable.combineLatest(latestNews, latestWeather, (news, weather) -> Pair(news, weather))
      .subscribe(item -> renderUi(item.first, item.second));

and am also unsure how you feel about using time operations in examples since you can't just run them and repro the behavior without blocking the thread somehow (TestObserver in unit tests or doing the work on a different thread, etc).

[akarnokd]

I prefer examples that can be tried quickly, thus examples requiring custom types are not good.

[ahmedre]

i am not happy with this example - any suggestions?

[akarnokd]

Have two intertwined intervals. The outer 1 seconds, the inner 100 milliseconds and the inner could be mapped as outer + "-" + inner so that the switch over to a different value is more clear.

[akarnokd]

I prefer examples that can be tried quickly, thus examples requiring custom types are not good.

[akarnokd]

Have two intertwined intervals. The outer 1 seconds, the inner 100 milliseconds and the inner could be mapped as outer + "-" + inner so that the switch over to a different value is more clear.

[akarnokd]

These descriptions are after Available and ReactiveX notes in #6131 btw.

[akarnokd]

Thanks for your contribution and keep up the good work! 👍

I'll merge this once the Travis CI service works properly again.

[codecov]

Codecov Report

Merging #6135 into 2.x will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6135      +/-   ##
============================================
- Coverage     98.31%   98.28%   -0.04%     
- Complexity     6192     6197       +5     
============================================
  Files           667      667              
  Lines         44853    44853              
  Branches       6213     6213              
============================================
- Hits          44097    44082      -15     
- Misses          228      232       +4     
- Partials        528      539      +11

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/flowable/FlowableFlatMapMaybe.java	91.78% <0%> (-3.87%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	94.87% <0%> (-3.42%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	95.52% <0%> (-1.5%)	2% <0%> (ø)
...ternal/operators/observable/ObservableFlatMap.java	87.22% <0%> (-0.96%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowableFlatMap.java	89.47% <0%> (-0.79%)	4% <0%> (ø)
... and 16 more

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