3.x: fix wording in Single javadocs

[akarnokd]

Some wording fixes in Single, includes #6565.

[codecov]

Codecov Report

Merging #6566 into 3.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@ Coverage Diff @@## 3.x #6566 +/- ## =========================================== + Coverage 98.19% 98.2% +0.01% - Complexity 6178 6180 +2  =========================================== Files 678 678 Lines 44886 44886 Branches 6212 6212 =========================================== + Hits 44074 44079 +5 + Misses 275 266 -9 - Partials 537 541 +4

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	148 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-2.62%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...java/io/reactivex/processors/UnicastProcessor.java	98.8% <0%> (-1.2%)	67% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.56% <0%> (-1.09%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.53% <0%> (-0.99%)	17% <0%> (-1%)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0%> (-0.86%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-0.75%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	94.89% <0%> (-0.73%)	2% <0%> (ø)
... and 21 more

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