docs(python): Update join_where docs to clarify behaviour

[adamreeve]

This is a small change to the join_where docs to add that this behaves like an inner join, and emphasise that row order isn't preserved.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.92%. Comparing base (5c4e7e9) to head (0bc8f7e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18670      +/-   ##
==========================================
+ Coverage   79.90%   79.92%   +0.01%     
==========================================
  Files        1512     1512              
  Lines      203203   203203              
  Branches     2891     2891              
==========================================
+ Hits       162379   162406      +27     
+ Misses      40274    40247      -27     
  Partials      550      550              

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