docs(python): Remove incorrect "1i (1 index count)" from some docs methods

[MarcoGorelli]

There are places where "1i" is allowed in the duration string language, but not always. .dt.offset_by('3i') for example doesn't make sense, because the operation can only be called on a temporal column anyway
join_asof already raises if '1i' is passed, I think the docs were just copy-and-pasted over

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.35%. Comparing base (96e1f01) to head (e2b40b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15750      +/-   ##
==========================================
- Coverage   81.35%   81.35%   -0.01%     
==========================================
  Files        1379     1379              
  Lines      176603   176603              
  Branches     2544     2544              
==========================================
- Hits       143677   143673       -4     
- Misses      32443    32447       +4     
  Partials      483      483              

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[stinodego]

Thanks!