clarify documentation of strip_whitespace

[micheleAlberto]

Change Summary

This pr bundles two changes:

• a documentation change suggested in Clarify documentation of strip_whitespace #9438 for StringConstraints strip_whitespace
• 2 tests that reflect the wording of the new documentations

Related issue number

fix #9438

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[micheleAlberto]

@sydney-runkle this is ready to review

[codspeed-hq]

CodSpeed Performance Report

Merging #9471 will not alter performance

_{Comparing micheleAlberto:clarify_docs_of_strip_whitespace (89c2a0c) with main (9030415)}

Summary

✅ 13 untouched benchmarks

[sydney-runkle]

Great, thanks!