2.x: improve JavaDocs of the subscribeActual methods

[akarnokd]

The PR improves the wording of the abstract subscribeActual method across the base reactive types.

[codecov]

Codecov Report

Merging #5981 into 2.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5981      +/-   ##
============================================
+ Coverage     98.23%   98.29%   +0.05%     
+ Complexity     6053     6050       -3     
============================================
  Files           656      656              
  Lines         44078    44078              
  Branches       6118     6118              
============================================
+ Hits          43299    43325      +26     
+ Misses          236      224      -12     
+ Partials        543      529      -14

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	115 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	535 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	557 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
... and 19 more

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