3.x: Prettify Flowable javadoc + validator

[akarnokd]

This PR unifies the Javadoc style of the Flowable type:

• Mention of Flowable has to be {@code Flowable}
• First mention of a set of types has to be {@link TheType}
• Subsequent mention of a set of types has to be {@code TheType}
• Some keywords and method names have to be {@code methodName}

In addition, JavadocWording has been modified to detect an additional set of wrong typename use.

[akarnokd]

Diff view due to a lot of changes: https://patch-diff.githubusercontent.com/raw/ReactiveX/RxJava/pull/6806.diff

[codecov]

Codecov Report

Merging #6806 into 3.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6806      +/-   ##
============================================
+ Coverage     98.22%   98.25%   +0.02%     
- Complexity     6387     6388       +1     
============================================
  Files           715      715              
  Lines         46496    46496              
  Branches       6371     6371              
============================================
+ Hits          45673    45684      +11     
+ Misses          298      291       -7     
+ Partials        525      521       -4

Impacted Files	Coverage Δ	Complexity Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100% <ø> (ø)	577 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.55% <0%> (-2.47%)	50% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-2.14%)	5% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-2.11%)	3% <0%> (ø)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	91.74% <0%> (-1.84%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	97.29% <0%> (-1.36%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-0.86%)	2% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-0.76%)	3% <0%> (ø)
... and 18 more

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