3.x: [Java 8] Add AutoCloseable <-> Disposable conversions, nicen docs

[akarnokd]

Add Disposables.fromAutoCloseable and Disposables.toAutoCloseable methods. Improve JavaDocs styling.

Resolves #6772
Resolves #6777

[JakeWharton]

You could also do

interface Disposable extends AutoCloseable {
  boolean isDisposed();
  void dispose();

  default void close() {
    dispose();
  }
}

[codecov]

Codecov Report

Merging #6780 into 3.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6780      +/-   ##
============================================
+ Coverage     98.22%   98.26%   +0.03%     
- Complexity     6236     6241       +5     
============================================
  Files           688      689       +1     
  Lines         45041    45053      +12     
  Branches       6193     6193              
============================================
+ Hits          44243    44270      +27     
+ Misses          280      273       -7     
+ Partials        518      510       -8

Impacted Files	Coverage Δ	Complexity Δ
...ctivex/rxjava3/disposables/RunnableDisposable.java	100% <ø> (ø)	3 <0> (ø)	⬇️
...eactivex/rxjava3/disposables/FutureDisposable.java	100% <ø> (ø)	6 <0> (ø)	⬇️
...x/rxjava3/disposables/AutoCloseableDisposable.java	100% <100%> (ø)	3 <3> (?)
...eactivex/rxjava3/disposables/ActionDisposable.java	100% <100%> (ø)	3 <1> (+1)	⬆️
.../io/reactivex/rxjava3/disposables/Disposables.java	100% <100%> (ø)	10 <2> (+2)	⬆️
...perators/single/SingleFlatMapIterableFlowable.java	94.16% <0%> (-2.5%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98.18% <0%> (-1.82%)	3% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
... and 17 more

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[akarnokd]

I've tried Disposable extends AutoCloseable before and ended up with hundreds of warnings about unclosed resources. I'd rather keep them separate.