2.x: Document replay(N) emission retention caveat

[BenTilbrook]

Adds javadocs indicating a caveat regarding size-bounded replay operators, subjects, and processors. See #5827 for details.

[codecov]

Codecov Report

Merging #5828 into 2.x will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5828      +/-   ##
============================================
- Coverage     96.44%   96.35%   -0.09%     
  Complexity     5817     5817              
============================================
  Files           634      634              
  Lines         41761    41761              
  Branches       5796     5796              
============================================
- Hits          40276    40239      -37     
- Misses          570      592      +22     
- Partials        915      930      +15

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	526 <0> (ø)	⬇️
...main/java/io/reactivex/subjects/ReplaySubject.java	97.33% <ø> (-0.23%)	49 <0> (ø)
.../java/io/reactivex/processors/ReplayProcessor.java	99.35% <ø> (ø)	51 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	509 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	91.03% <0%> (-4.94%)	60% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...internal/disposables/ArrayCompositeDisposable.java	96.42% <0%> (-3.58%)	15% <0%> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	91.8% <0%> (-3.28%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	61.8% <0%> (-2.78%)	35% <0%> (-2%)
... and 29 more

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