Add cryptographic security note to System.Random API docs

[Copilot]

The main System.Random API reference lacked the security caveat present in the supplementary docs, leaving users unaware that System.Random is unsuitable for cryptographic use.

Changes

• xml/System/Random.xml: Expanded the type-level <remarks> from a bare supplementary-docs link into a <format type="text/markdown"> block containing:
  • A [!NOTE] callout directing users to System.Security.Cryptography.RandomNumberGenerator for cryptographically secure random numbers
  • The existing supplementary docs link (reformatted to markdown link syntax)

The rendered note will read:

Note
To generate a cryptographically secure random number, such as one that's suitable for creating a random password, use one of the static methods in the System.Security.Cryptography.RandomNumberGenerator class.

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Tagging subscribers to this area: @dotnet/area-system-security, @vcsjones

[Copilot]

Pull request overview

Adds a cryptographic safety caveat to the System.Random API reference documentation so users aren’t misled into using Random for security-sensitive scenarios.

Changes:

• Replaced the one-line <remarks> with a markdown <format> block.
• Added a [!NOTE] callout pointing users to System.Security.Cryptography.RandomNumberGenerator for cryptographically secure randomness.
• Reformatted the existing supplemental remarks link as a markdown link.