docs: Add missing deprecation notice to str.concat

[LiamConnors]

#19144

Would it make sense to take out the deprecation notice further down as part of this? I guess it wouldn't change for str.concat now if it's been deprecated?

polars/py-polars/polars/expr/string.py

Lines 2776 to 2780 in 133bf47

	issue_deprecation_warning(
	"The default `delimiter` for `str.concat` will change from '-' to an empty string."
	" Pass a delimiter to silence this warning.",
	version="0.20.5",
	)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.22%. Comparing base (dbc8368) to head (1351550).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #19324      +/-   ##
==========================================
- Coverage   80.24%   80.22%   -0.02%     
==========================================
  Files        1523     1523              
  Lines      209524   209525       +1     
  Branches     2434     2434              
==========================================
- Hits       168127   168093      -34     
- Misses      40842    40877      +35     
  Partials      555      555              

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