2.x: Add blockingSubscribe JavaDoc clarifications

[akarnokd]

This PR adds some clarifications to the blockingSubscribe methods on Flowable and Observable.

[codecov]

Codecov Report

Merging #5984 into 2.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5984      +/-   ##
============================================
+ Coverage     98.26%   98.26%   +<.01%     
+ Complexity     6053     6051       -2     
============================================
  Files           656      656              
  Lines         44078    44078              
  Branches       6118     6118              
============================================
+ Hits          43313    43314       +1     
  Misses          228      228              
+ Partials        537      536       -1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	557 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	535 <1> (ø)	⬇️
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	94.02% <0%> (-2.24%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-2.16%)	9% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	97.44% <0%> (-1.71%)	11% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
... and 18 more

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