docs(python): Update DataFrame join examples

[SamuelAllain]

coalesce=True has no effect when how="left" so it should be demonstrated with how="full" instead. I kept an example of how="left" (without coalesce=True) because it might be usefull too. I don't know Rust but the doc may be adapted there too

[alexander-beedie]

Good one, thx ;)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.96%. Comparing base (04004b8) to head (0400b58).
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- Coverage   78.97%   78.96%   -0.01%     
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  Files        1564     1564              
  Lines      220807   220807              
  Branches     2510     2510              
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- Hits       174377   174370       -7     
- Misses      45855    45862       +7     
  Partials      575      575              

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