3.x: Update the javadoc description of cast()

[akarnokd]

Reword the operator description to avoid some confusion how cast() works.

Resolves #7605

The related images had mistakes too and have been updated in the wiki section separately:

]

[codecov]

Codecov Report

Merging #7606 (555e196) into 3.x (e6f152d) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7606      +/-   ##
============================================
- Coverage     99.60%   99.58%   -0.02%     
- Complexity     6801     6802       +1     
============================================
  Files           752      752              
  Lines         47707    47707              
  Branches       6401     6401              
============================================
- Hits          47520    47511       -9     
- Misses           85       89       +4     
- Partials        102      107       +5     

Files	Coverage Δ
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)

... and 9 files with indirect coverage changes

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