2.x: Expand {X}Processor JavaDocs by syncing with {X}Subject docs

[akarnokd]

This PR expands and adapts the JavaDocs of AsyncProcessor, BehaviorProcessor, PublishProcessor, ReplayProcessor and UnicastProcessor with the details of their already documented Subject variants.

In addition, some wording has been fixed with UnicastSubject as it was refering to plural Observers in some sentences even though it only supports one.

Since the basis of this expansion is copy-paste, please read through the changes carefully to verify the terminology and described behavior matches what's usually expected from Flowables/FlowableProcessors (i.e., subjects don't have to deal with backpressure and thus nothing much to elaborate on that in a subject doc).

[akarnokd]

Moved to the top after the first sentence.

[codecov]

Codecov Report

Merging #6054 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6054      +/-   ##
============================================
- Coverage     98.29%   98.28%   -0.01%     
+ Complexity     6193     6190       -3     
============================================
  Files           666      666              
  Lines         44802    44802              
  Branches       6206     6206              
============================================
- Hits          44039    44035       -4     
- Misses          229      232       +3     
- Partials        534      535       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
.../java/io/reactivex/processors/ReplayProcessor.java	99.58% <ø> (ø)	52 <0> (ø)	⬇️
...n/java/io/reactivex/processors/AsyncProcessor.java	99.08% <ø> (ø)	52 <0> (ø)	⬇️
...java/io/reactivex/processors/PublishProcessor.java	100% <ø> (ø)	43 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	99.1% <ø> (ø)	60 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	539 <0> (ø)	⬇️
...ain/java/io/reactivex/subjects/UnicastSubject.java	100% <ø> (ø)	65 <0> (ø)	⬇️
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <ø> (-1.2%)	67 <0> (-1)
...l/operators/observable/ObservableFlatMapMaybe.java	91.5% <0%> (-5.23%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	95.83% <0%> (-4.17%)	55% <0%> (-3%)
... and 22 more

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