2.x: Update marble diagrams for sample overloads

[akarnokd]

This PR updates the referenced marble diagrams to reflect the actual behavior and options with sample.

New images:

• sample(time, emitLast)

• sample(time, scheduler, emitLast)

• sample(other) - the original image indicated the emission of the last item which was not the case

• sample(other, emitLast)

[codecov-io]

Current coverage is 95.48% (diff: 100%)

Merging #4990 into 2.x will decrease coverage by 0.04%

@@                2.x      #4990   diff @@
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  Files           592        592          
  Lines         37969      37969          
  Methods           0          0          
  Messages          0          0          
  Branches       5752       5752          
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- Hits          36273      36256    -17   
- Misses          741        753    +12   
- Partials        955        960     +5   

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[akarnokd]

switchIfEmpty:

Maybe:

Maybe.switchIfEmpty

[DavidMGross]

FWIW, when I last had time to work on the marble diagrams, I began using this iconography for the various new observable variants:

This is reflected in some of the existing marble diagrams.

[akarnokd]

Those tail symbols look odd to me and look easy to miss or categorize as diagram errors.