docs: clarify req.params type and usage with regular expressions

[krzysdz]

Fixes #2100

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[github-actions]

🚦 Lighthouse Results (Mobile & Desktop)

URL	Device	Perf	A11y	Best Practices
/	mobile	🔴 58	🟢 100	🟢 96
/en/blog/posts.html	mobile	🟢 98	🟢 96	🟢 96
/en/5x/api.html	mobile	🟠 76	🟢 95	🟢 96
/	desktop	🟢 100	🟢 100	🟢 96
/en/blog/posts.html	desktop	🟢 100	🟢 96	🟢 92
/en/5x/api.html	desktop	🟢 97	🟢 95	🟢 96

[bjohansebas]

Good catch, for referencese expressjs/express#1835 is where the change was made for express v4/v5

ref https://github.com/pillarjs/router/blob/cb2a5fcdf31b9cc92296183ceeff8126b5c57453/index.js#L68

[Copilot]

Pull request overview

This PR clarifies the documentation for req.params to better explain how it handles regular expression route patterns. The changes improve technical accuracy by correcting a misleading description and add helpful information about named capturing groups.

• Corrects the description of how capture groups are stored (as integer keys in an object, not in an array)
• Adds documentation for named capturing groups in regular expressions

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
_includes/api/en/5x/req-params.md	Updates Express 5.x documentation to clarify that regex capture groups use integer keys and adds info about named capturing groups
_includes/api/en/4x/req-params.md	Updates Express 4.x documentation with the same clarifications for consistency

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