3.x: Add 'Error handling' section to Observable.blockingFirst JavaDocs

[vjgarciag96]

Add missing JavaDocs section.

Resolves #6900

[codecov]

Codecov Report

Merging #6903 into 3.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                3.x   #6903      +/-   ##
===========================================
+ Coverage     98.25%   98.3%   +0.05%     
+ Complexity     6620    6616       -4     
===========================================
  Files           742     742              
  Lines         47426   47426              
  Branches       6443    6443              
===========================================
+ Hits          46597   46623      +26     
+ Misses          307     287      -20     
+ Partials        522     516       -6

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <ø> (ø)	565 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	90.47% <0%> (-6.35%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-1.71%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	95.9% <0%> (-0.82%)	2% <0%> (ø)
... and 15 more

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