2.x: small note on Maybe.defaultIfEmpty regarding toSingle

[akarnokd]

Ths PR replaces #5485 regarding the issue of #5480 because our javadoc validation did not allow that type of wording (i.e., mentioning Single when the method signature is not related to it).

[codecov]

Codecov Report

Merging #5486 into 2.x will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5486      +/-   ##
============================================
+ Coverage     96.01%   96.08%   +0.07%     
+ Complexity     5803     5799       -4     
============================================
  Files           631      631              
  Lines         41297    41297              
  Branches       5743     5743              
============================================
+ Hits          39650    39682      +32     
+ Misses          655      634      -21     
+ Partials        992      981      -11

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	168 <0> (ø)	⬇️
...tivex/internal/schedulers/InstantPeriodicTask.java	61.11% <0%> (-5.56%)	7% <0%> (-2%)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...rnal/operators/completable/CompletableTimeout.java	94% <0%> (-4%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...ternal/operators/completable/CompletableCache.java	96.96% <0%> (-3.04%)	23% <0%> (-1%)
...tivex/internal/schedulers/TrampolineScheduler.java	92.4% <0%> (-2.54%)	6% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-2.18%)	2% <0%> (ø)
...rnal/operators/observable/ObservableObserveOn.java	97.95% <0%> (-2.05%)	3% <0%> (ø)
... and 40 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 46ec6a6...f7c80e7. Read the comment docs.