Reword some regex safety warnings

[GrabYourPitchforks]

I'm updating regex docs across the various docs repos. This is one part of that update.

Changes:

• Remove misleading guidance that timeout values are appropriate ways to guard against all hostile values.
• Consolidate the warnings to the Regex type rather than just two specific constructors.
• Update external link from CISA to OWASP. The CISA guidance is primarily DDoS-related, which isn't quite relevant to the discussion here. OWASP's discussion is geared specifically toward ReDoS, which is on point.

Related PRs:

• Fix incorrect usage of regex in WPF sample docs-desktop#2250
• Reword some regex safety warnings docs#54621
• Reword some regex safety warnings #12848 (this one)

[Copilot]

Pull request overview

This PR updates the .NET API reference documentation to present more accurate guidance about regular expression safety, consolidating the primary warning at the Regex type level and updating an external reference to a ReDoS-focused resource.

Changes:

• Converts Regex type-level remarks to Markdown and adds a centralized warning about using Regex with untrusted input, linking to OWASP ReDoS guidance.
• Removes the prior constructor-level warning text that implied timeouts are a sufficient defense against all hostile inputs.
• Tweaks RegexOptions.NonBacktracking summary text to point readers to the “Non-Backtracking mode” section of the options article.

Reviewed changes

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

File	Description
xml/System.Text.RegularExpressions/RegexOptions.xml	Clarifies the NonBacktracking option summary with a more specific docs pointer.
xml/System.Text.RegularExpressions/Regex.xml	Centralizes and rewords regex safety guidance in the type-level remarks and removes redundant constructor warnings.

[gewarren]

I don't know why the GitOps policy we have to prevent edits to these files didn't fire, but System.Text.RegularExpressions has its source of truth for docs in the dotnet/runtime repo as of a few weeks ago. So please make these changes there instead: https://github.com/dotnet/runtime/blob/0a8142e2de2a05ac170dac8e9edffb41d059463b/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Regex.cs#L24