3.x: fromRunnable/fromAction javadoc improvements

[akarnokd]

• Fix the wording across the base types.
• Add missing error handling section.
• Point to fromAction in fromRunnable docs.

[codecov]

Codecov Report

Merging #7071 into 3.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7071      +/-   ##
============================================
- Coverage     99.55%   99.51%   -0.05%     
  Complexity     6669     6669              
============================================
  Files           742      742              
  Lines         47275    47275              
  Branches       6374     6374              
============================================
- Hits          47066    47045      -21     
- Misses           96      105       +9     
- Partials        113      125      +12     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100.00% <ø> (ø)	140.00 <0.00> (ø)
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)	595.00 <0.00> (ø)
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100.00% <ø> (ø)	214.00 <0.00> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)	565.00 <0.00> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	97.48% <0.00%> (-2.52%)	50.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0.00%> (-2.33%)	2.00% <0.00%> (ø%)
.../internal/disposables/ListCompositeDisposable.java	98.00% <0.00%> (-2.00%)	34.00% <0.00%> (-1.00%)
...a3/internal/operators/flowable/FlowableCreate.java	97.41% <0.00%> (-1.62%)	6.00% <0.00%> (ø%)
...ternal/operators/observable/ObservableGroupBy.java	98.90% <0.00%> (-1.10%)	2.00% <0.00%> (ø%)
... and 12 more

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