Backport missing Regex constructor remarks and examples

[Copilot]

System.Text.RegularExpressions constructor docs were missing remarks and examples that had already been backported elsewhere in the API docs flow. This updates the runtime source docs so the constructor guidance and examples live with the source of truth.

• Documentation backport

  • Added missing remarks content to the three public Regex constructors in /home/runner/work/runtime/runtime/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Regex.cs.
  • Restored constructor-specific guidance around pattern behavior, case sensitivity, and match timeout usage.

• Example coverage

  • Added snippet-backed examples in /home/runner/work/runtime/runtime/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Examples.cs for:
    • new Regex(string)
    • new Regex(string, RegexOptions)
    • new Regex(string, RegexOptions, TimeSpan)
  • Wired the constructor docs to those validated snippets using the existing doc example pattern.

• Example stability

  • Kept the timeout example behaviorally equivalent while reducing the adversarial input length so the example remains suitable for slower test environments.

string pattern = @"\b[at]\w+\b";
string input = "The archive was trimmed and tagged.";
MatchCollection matches = new Regex(pattern, RegexOptions.IgnoreCase).Matches(input);

foreach (Match match in matches)
    Console.WriteLine(match.Value);

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