2.x: Improve the wording of the Maybe.fromCallable JavaDoc

[akarnokd]

This PR improves the wording and details of the Maybe.fromCallable operator's JavaDoc.

[codecov]

Codecov Report

Merging #5848 into 2.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5848      +/-   ##
============================================
- Coverage     96.43%   96.37%   -0.06%     
+ Complexity     5816     5815       -1     
============================================
  Files           634      634              
  Lines         41760    41760              
  Branches       5796     5796              
============================================
- Hits          40270    40248      -22     
- Misses          571      587      +16     
- Partials        919      925       +6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	171 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	89.85% <0%> (-6.77%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	86.24% <0%> (-5.83%)	54% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...activex/internal/operators/single/SingleCache.java	97.05% <0%> (-2.95%)	23% <0%> (-1%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	61.8% <0%> (-2.78%)	35% <0%> (-2%)
...x/internal/operators/flowable/FlowablePublish.java	93.83% <0%> (-1.77%)	11% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-1.5%)	2% <0%> (ø)
...ivex/internal/operators/maybe/MaybeMergeArray.java	96.62% <0%> (-1.13%)	6% <0%> (ø)
... and 23 more

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