2.x: document (internal) SimpleQueue

[akarnokd]

The interface was missing javadoc on its methods.

[codecov]

Codecov Report

Merging #5056 into 2.x will increase coverage by 0.19%.

@@             Coverage Diff              @@
##                2.x    #5056      +/-   ##
============================================
+ Coverage     95.57%   95.76%   +0.19%     
- Complexity     5542     5548       +6     
============================================
  Files           612      612              
  Lines         39576    39576              
  Branches       5553     5553              
============================================
+ Hits          37824    37901      +77     
+ Misses          764      723      -41     
+ Partials        988      952      -36

Impacted Files	Coverage Δ	Complexity Δ
...al/operators/observable/ObservableSampleTimed.java	93.33% <ø> (-5%)	3% <ø> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <ø> (-4.62%)	27% <ø> (-1%)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	97.95% <ø> (-2.05%)	2% <ø> (ø)
...ternal/operators/flowable/FlowableSampleTimed.java	95.58% <ø> (-1.48%)	3% <ø> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	92.04% <ø> (-1.14%)	24% <ø> (-1%)
...main/java/io/reactivex/subjects/ReplaySubject.java	96.44% <ø> (-0.67%)	50% <ø> (-1%)
...ternal/operators/observable/ObservableFlatMap.java	82.78% <ø> (-0.67%)	3% <ø> (ø)
...ex/internal/operators/flowable/FlowableReplay.java	89.79% <ø> (-0.47%)	17% <ø> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	95.93% <ø> (ø)	6% <ø> (ø)	❌
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <ø> (ø)	5% <ø> (ø)	❌
... and 32 more

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