2.x: Improve BehaviorProcessor JavaDoc

[akarnokd]

A StackOverflow question prompted me to look at BehaviorProcessor to find it quite lacking of a detailed JavaDoc. This PR adds sentences and examples (I could think of) to it that explain the available features and behavior of this Processor implementation.

If the sentences are reviewed, the BehaviorSubject will be updated with the applicable information (i.e., everything minus backpressure related sentences).

[codecov]

Codecov Report

Merging #5778 into 2.x will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5778      +/-   ##
============================================
+ Coverage     96.25%   96.33%   +0.07%     
  Complexity     5810     5810              
============================================
  Files           634      634              
  Lines         41607    41607              
  Branches       5770     5770              
============================================
+ Hits          40049    40082      +33     
+ Misses          629      600      -29     
+ Partials        929      925       -4

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/reactivex/processors/BehaviorProcessor.java	95.06% <ø> (+4.03%)	59 <0> (-1)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-7.19%)	2% <0%> (ø)
...a/io/reactivex/internal/queue/MpscLinkedQueue.java	98.03% <0%> (-1.97%)	17% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
...vex/internal/subscribers/QueueDrainSubscriber.java	50% <0%> (-1.57%)	15% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	94.71% <0%> (-1.33%)	11% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.1% <0%> (-0.88%)	10% <0%> (-1%)
...nternal/operators/parallel/ParallelSortedJoin.java	94.92% <0%> (-0.73%)	2% <0%> (ø)
...nternal/operators/observable/ObservableReplay.java	98.05% <0%> (-0.56%)	23% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	97.1% <0%> (-0.49%)	8% <0%> (ø)
... and 21 more

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