2.x: Fix Completable mergeX JavaDoc missing dt before dd

[akarnokd]

Couple of the <dt><b>Error handling:</b></dt> subheaders were missing from various Completable.mergeX operators in the JavaDocs.

[codecov]

Codecov Report

Merging #6103 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6103      +/-   ##
============================================
+ Coverage     98.24%   98.25%   +0.01%     
  Complexity     6197     6197              
============================================
  Files           667      667              
  Lines         44862    44862              
  Branches       6213     6213              
============================================
+ Hits          44075    44081       +6     
+ Misses          243      236       -7     
- Partials        544      545       +1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	117 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	85.62% <0%> (-6.54%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
... and 24 more

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