2.x: Expand the documentation of the Flowable.lift() operator

[akarnokd]

This PR expands the documentation of the Flowable.lift() operator used for inserting custom operators into flows in a functional and fluent fashion.

The other reactive base classes feature similar lift() operators for which the text can be adapted and contributed in a separate PR once the content of this PR is stabilized by review feedback.

Related: #5862

[codecov]

Codecov Report

Merging #5863 into 2.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5863      +/-   ##
============================================
+ Coverage     96.51%   96.54%   +0.02%     
+ Complexity     5856     5855       -1     
============================================
  Files           646      646              
  Lines         42640    42640              
  Branches       5910     5910              
============================================
+ Hits          41156    41166      +10     
+ Misses          574      567       -7     
+ Partials        910      907       -3

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	533 <0> (ø)	⬇️
...nternal/operators/parallel/ParallelReduceFull.java	91.17% <0%> (-3.93%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <0%> (-1.35%)	7% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
...ex/internal/operators/flowable/FlowableReplay.java	90.06% <0%> (-0.85%)	22% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
... and 19 more

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[RobLewis]

Thank you. I found the documentation much improved and was able to use it to build a working example operator (I may post it as a Gist if anyone is interested).