2.x: Expand the Javadoc of Flowable

[akarnokd]

• Add links to Reactive Streams
• Fix hyphenation
• Turn some @code into @links.
• Add a short protocol description
• Add basic example usage
• Add example to use create for custom sources
• Mention the synchronous/sequential nature and link in some operators.

[codecov]

Codecov Report

Merging #6506 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #6506      +/-   ##
===========================================
+ Coverage     98.26%   98.3%   +0.03%     
- Complexity     6295    6297       +2     
===========================================
  Files           675     675              
  Lines         45163   45163              
  Branches       6244    6244              
===========================================
+ Hits          44380   44397      +17     
+ Misses          248     237      -11     
+ Partials        535     529       -6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	567 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	542 <0> (ø)	⬇️
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	95.42% <0%> (-1.31%)	2% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.75% <0%> (-0.95%)	3% <0%> (ø)
... and 20 more

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