Expand Observable#debounce and Flowable#debounce javadoc

[RomanWuattier]

Mention that if the processing of a task takes too long
and a newer item arrives then the previous task will
get disposed interrupting a long running work.

Fixes: #6288

[akarnokd]

The scheduler wording needs to be adapted to the particular overload.

[codecov]

Codecov Report

Merging #6377 into 2.x will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6377      +/-   ##
============================================
- Coverage     98.25%   98.16%   -0.09%     
+ Complexity     6292     6286       -6     
============================================
  Files           673      673              
  Lines         45092    45092              
  Branches       6239     6239              
============================================
- Hits          44306    44266      -40     
- Misses          248      274      +26     
- Partials        538      552      +14

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	567 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	542 <0> (ø)	⬇️
...ex/internal/operators/flowable/FlowableCreate.java	90% <0%> (-7.1%)	6% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	91.5% <0%> (-4.58%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	90.42% <0%> (-3.2%)	9% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...ex/internal/subscriptions/SubscriptionArbiter.java	97.47% <0%> (-2.53%)	50% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
... and 27 more

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[RomanWuattier]

Thanks for the review. Done.

[RomanWuattier]

Thanks, the wording for selector-based debounces is updated.