2.x: Fix Completable.toMaybe() @return javadoc.

[artem-zinnatullin]

Current javadoc:

@return a {@link Maybe} that emits a single item T or an error.

But in reality, converting Maybe to Completable should never emit item T.

[codecov]

Codecov Report

Merging #5936 into 2.x will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5936      +/-   ##
============================================
- Coverage     98.25%   98.24%   -0.02%     
+ Complexity     6019     6018       -1     
============================================
  Files           656      656              
  Lines         44040    44040              
  Branches       6102     6102              
============================================
- Hits          43273    43268       -5     
+ Misses          230      227       -3     
- Partials        537      545       +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	115 <0> (ø)	⬇️
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-2.5%)	2% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
.../operators/mixed/FlowableConcatMapCompletable.java	98.29% <0%> (-1.71%)	2% <0%> (ø)
...internal/operators/flowable/FlowableFromArray.java	94.91% <0%> (-1.7%)	3% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.27% <0%> (-1.45%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	99.11% <0%> (-0.89%)	10% <0%> (-1%)
... and 24 more

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