2.x: blockingX JavaDoc to mention wrapping of checked Exceptions

[akarnokd]

This PR adds an Error handling section to various blockingX operators to specify checked exceptions from their source will be wrapped into RuntimeExceptions.

[codecov]

Codecov Report

Merging #5951 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5951      +/-   ##
============================================
+ Coverage     98.27%   98.31%   +0.03%     
  Complexity     6020     6020              
============================================
  Files           656      656              
  Lines         44036    44036              
  Branches       6100     6100              
============================================
+ Hits          43278    43295      +17     
+ Misses          227      216      -11     
+ Partials        531      525       -6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	142 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	535 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	115 <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)	⬇️
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...ternal/operators/observable/ObservableFlatMap.java	86.26% <0%> (-1.6%)	3% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.26% <0%> (-0.87%)	2% <0%> (ø)
... and 25 more

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