2.x: add a couple of @see to Completable

[akarnokd]

This PR adds a couple of @see cross references to Completable JavaDoc.

[codecov]

Codecov Report

Merging #5758 into 2.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5758      +/-   ##
============================================
- Coverage     96.31%   96.27%   -0.05%     
- Complexity     5838     5840       +2     
============================================
  Files           634      634              
  Lines         41653    41653              
  Branches       5769     5769              
============================================
- Hits          40118    40101      -17     
- Misses          603      616      +13     
- Partials        932      936       +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	115 <0> (ø)	⬇️
...ava/io/reactivex/processors/BehaviorProcessor.java	90.26% <0%> (-5.76%)	61% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-2.64%)	11% <0%> (+1%)
...l/operators/observable/ObservableFlatMapMaybe.java	87.58% <0%> (-2.62%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...ex/internal/subscribers/InnerQueuedSubscriber.java	96.07% <0%> (-1.97%)	18% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	98.3% <0%> (-1.7%)	11% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-1.64%)	2% <0%> (ø)
...nal/operators/flowable/FlowableConcatMapEager.java	97.1% <0%> (-1.16%)	2% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	96.61% <0%> (-0.97%)	8% <0%> (ø)
... and 18 more

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