Fix NotRequired qualifier not taken into account in stringified annotation

[Viicos]

Also update docstring of the relevant GenerateSchema method.

Regression in #11479.

Fixes #11557, fixes pydantic/pydantic-ai#1143.

Change Summary

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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

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_generate_schema.py
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[codspeed-hq]

CodSpeed Performance Report

Merging #11559 will not alter performance

_{Comparing td-notrequired (5970ea2) with main (b9fb3f1)}

Summary

✅ 46 untouched benchmarks