2.x: better documentation on the abstract consumer classes

[akarnokd]

This PR improves the documentation of the abstract consumer classes such as DisposableSubscriber, ResourceSubscriber and their counterparts for the other base reactive types.

It contains a lot of copy-paste so please read through all of the text in case the text was not properly adapted to the abstract class at hand.

Related: #5148.

[codecov]

Codecov Report

Merging #5210 into 2.x will increase coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #5210      +/-   ##
===========================================
+ Coverage     95.9%   96.01%   +0.11%     
- Complexity    5678     5684       +6     
===========================================
  Files          621      621              
  Lines        40611    40611              
  Branches      5632     5632              
===========================================
+ Hits         38947    38992      +45     
+ Misses         673      644      -29     
+ Partials       991      975      -16

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/reactivex/observers/DisposableObserver.java	100% <ø> (ø)	7 <0> (ø)	⬇️
...o/reactivex/observers/DisposableMaybeObserver.java	100% <ø> (ø)	7 <0> (ø)	⬇️
...io/reactivex/observers/ResourceSingleObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
...tivex/observers/DisposableCompletableObserver.java	100% <ø> (ø)	7 <0> (ø)	⬇️
.../reactivex/observers/DisposableSingleObserver.java	100% <ø> (ø)	7 <0> (ø)	⬇️
...va/io/reactivex/subscribers/DefaultSubscriber.java	86.66% <ø> (ø)	5 <0> (ø)	⬇️
...activex/observers/ResourceCompletableObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
...io/reactivex/subscribers/DisposableSubscriber.java	100% <ø> (ø)	9 <0> (ø)	⬇️
.../java/io/reactivex/observers/ResourceObserver.java	100% <ø> (ø)	8 <0> (ø)	⬇️
...a/io/reactivex/subscribers/ResourceSubscriber.java	100% <ø> (ø)	9 <0> (ø)	⬇️
... and 47 more

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