3.x: Improve Javadocs of Completable and some others

[akarnokd]

Improve the JavaDocs of Completable by

• Using {@link} and {@code} tags
• Adding @throws regarding parameter validation
• Fix wording of some operators

[codecov]

Codecov Report

Merging #6809 into 3.x will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6809      +/-   ##
============================================
- Coverage     98.28%   98.26%   -0.03%     
+ Complexity     6388     6387       -1     
============================================
  Files           715      715              
  Lines         46496    46498       +2     
  Branches       6371     6371              
============================================
- Hits          45698    45689       -9     
- Misses          281      288       +7     
- Partials        517      521       +4

Impacted Files	Coverage Δ	Complexity Δ
...vex/rxjava3/observables/ConnectableObservable.java	100% <ø> (ø)	12 <0> (ø)	⬇️
...in/java/io/reactivex/rxjava3/core/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...activex/rxjava3/flowables/ConnectableFlowable.java	100% <ø> (ø)	12 <0> (ø)	⬇️
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.55% <ø> (-2.47%)	50 <0> (ø)
...a/io/reactivex/rxjava3/disposables/Disposable.java	100% <100%> (ø)	9 <2> (ø)	⬇️
...activex/rxjava3/processors/MulticastProcessor.java	98.25% <100%> (-0.44%)	83 <2> (-1)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100% <100%> (ø)	553 <1> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	86.61% <0%> (-7.05%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	89.68% <0%> (-4.77%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
... and 26 more

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