2.x: improve package JavaDoc

[akarnokd]

Add Maybe to the io.reactivex package Javadoc. Reword io.reactivex.observers and io.reactivex.subscribers as there are no utility classes in 2.x in them to create consumers from callbacks (those are internal and offered by the subscribe() overloads only).

[codecov]

Codecov Report

Merging #5648 into 2.x will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5648      +/-   ##
============================================
- Coverage     96.32%   96.25%   -0.08%     
+ Complexity     5851     5849       -2     
============================================
  Files           634      634              
  Lines         41539    41539              
  Branches       5752     5752              
============================================
- Hits          40014    39983      -31     
- Misses          604      609       +5     
- Partials        921      947      +26

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableFlatMapSingle.java	90.29% <0%> (-5.23%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	88.88% <0%> (-4.58%)	2% <0%> (ø)
...erators/completable/CompletableConcatIterable.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	61.53% <0%> (-2.57%)	12% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	94.27% <0%> (-1.77%)	11% <0%> (ø)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.02% <0%> (-1.64%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.68% <0%> (-1.45%)	2% <0%> (ø)
... and 15 more

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