2.x: Explain the properties of the XEmitter interfaces in detail

[akarnokd]

This PR explains the properties of the 5 specifc Emitter types in more details:

• error handling
• resource handling
• concurrent use aspects
• the difference between Disposable and Cancellable resource handling
• terminal state effects

[codecov]

Codecov Report

Merging #5844 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5844      +/-   ##
============================================
+ Coverage     96.38%   96.38%   +<.01%     
- Complexity     5812     5815       +3     
============================================
  Files           634      634              
  Lines         41760    41760              
  Branches       5796     5796              
============================================
+ Hits          40251    40252       +1     
+ Misses          591      583       -8     
- Partials        918      925       +7

Impacted Files	Coverage Δ	Complexity Δ
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-3.51%)	10% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	87.58% <0%> (-1.97%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	97.56% <0%> (-1.63%)	3% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	96.67% <0%> (-1.34%)	49% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...operators/flowable/FlowableWithLatestFromMany.java	99.24% <0%> (-0.76%)	7% <0%> (ø)
... and 19 more

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