3.x: Improve JavaDocs of Maybe

[akarnokd]

Improve the javadocs by:

• Adding {@code} and {@link} tags
• Fixed wording and copy-paste mistakes
• Added @throws for the parameter validation exceptions

[codecov]

Codecov Report

Merging #6815 into 3.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6815      +/-   ##
============================================
+ Coverage     98.23%   98.26%   +0.03%     
- Complexity     6376     6377       +1     
============================================
  Files           715      715              
  Lines         46486    46486              
  Branches       6371     6371              
============================================
+ Hits          45666    45681      +15     
+ Misses          293      289       -4     
+ Partials        527      516      -11

Impacted Files	Coverage Δ	Complexity Δ
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <ø> (ø)	151 <0> (ø)	⬇️
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	179 <0> (ø)	⬇️
.../operators/observable/ObservableFlatMapSingle.java	91.26% <0%> (-1.59%)	2% <0%> (ø)
...ivex/rxjava3/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.75% <0%> (-1.05%)	2% <0%> (ø)
...3/internal/operators/flowable/FlowablePublish.java	97.53% <0%> (-0.99%)	17% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	92.25% <0%> (-0.71%)	2% <0%> (ø)
.../io/reactivex/rxjava3/subjects/UnicastSubject.java	98.64% <0%> (-0.68%)	61% <0%> (-1%)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.56%)	3% <0%> (ø)
... and 16 more

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