2.x: Improve the JavaDoc of the other lift() operators

[akarnokd]

Improve the JavaDoc of the {Observable | Maybe | Single | Completable}.lift() operator, adapting the text from #5863.

[codecov]

Codecov Report

Merging #5865 into 2.x will decrease coverage by 0.08%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5865      +/-   ##
============================================
- Coverage     96.55%   96.46%   -0.09%     
+ Complexity     5858     5853       -5     
============================================
  Files           646      646              
  Lines         42640    42640              
  Branches       5910     5910              
============================================
- Hits          41170    41134      -36     
- Misses          567      586      +19     
- Partials        903      920      +17

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	142 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	515 <0> (ø)	⬇️
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	533 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	115 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-6.72%)	2% <0%> (ø)
.../io/reactivex/internal/functions/ObjectHelper.java	94.73% <0%> (-5.27%)	20% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-4.35%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
... and 26 more

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