docs(python): Remove y and mo from join_asof

[deanm0000]

Fixes #18955

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 79.86%. Comparing base (71a8b05) to head (d496dbe).
Report is 45 commits behind head on main.

Files with missing lines	Patch %	Lines
py-polars/polars/_utils/convert.py	25.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19027      +/-   ##
==========================================
- Coverage   79.88%   79.86%   -0.02%     
==========================================
  Files        1522     1524       +2     
  Lines      206958   207753     +795     
  Branches     2906     2910       +4     
==========================================
+ Hits       165322   165924     +602     
- Misses      41088    41278     +190     
- Partials      548      551       +3     

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[MarcoGorelli]

thanks for looking into this

• should the validation be done in rust?
• if month and year are being removed, so should quarter