2.x: Inline CompositeDisposable JavaDoc

[akarnokd]

Normally, overriding a method will reuse the JavaDoc unless specified directly on the new method. Unfortunately, when the base type is part of a javadoc exclude, such as everything below **/internal, the documentation is not copied and public facing methods have no HTML documentation:

This PR copies the javadoc of the internal DisposableContainer onto CompositeDisposable so it shows up properly:

The IDEs still show the documentation correctly.

[codecov]

Codecov Report

Merging #6031 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #6031      +/-   ##
===========================================
- Coverage     98.3%   98.28%   -0.03%     
+ Complexity    6175     6174       -1     
===========================================
  Files          665      665              
  Lines        44729    44729              
  Branches      6205     6205              
===========================================
- Hits         43969    43960       -9     
- Misses         226      235       +9     
  Partials       534      534

Impacted Files	Coverage Δ	Complexity Δ
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <ø> (-1.86%)	39 <0> (-1)
...l/operators/observable/ObservableFlatMapMaybe.java	85.62% <0%> (-7.19%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...internal/operators/completable/CompletableAmb.java	96.61% <0%> (-3.39%)	10% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-1.45%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-1.09%)	2% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
... and 19 more

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