docs(python): Remove note about guaranteed left join order

[stinodego]

Ref #19576

We currently guarantee preserving the order on a left join. In the future, we will no longer guarantee this by default and add a maintain_order argument to control this, as guaranteeing the order can be expensive under certain conditions.

So I'm removing the note about the guarantee from the docs at this time. The order will still be preserved for now, but users should not blindly rely on this as it will break in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.50%. Comparing base (83b5042) to head (efd24ec).
Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #20048   +/-   ##
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  Coverage   79.50%   79.50%           
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  Files        1562     1562           
  Lines      216943   216943           
  Branches     2459     2459           
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+ Hits       172477   172487   +10     
+ Misses      43907    43897   -10     
  Partials      559      559           

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