Correct None enum description

[joynoele]

Correct erroneous documentation description of the Enum CngExportPolicies.None option.
Correction text is kept as simple as possible. Conclusion reached via my own testing, the two open issues with the same question, and reply from reply from member @bartonjs:

CngExportPolicy.None is "non-exportable". If you're creating a key with CngKey.Create and don't specify an export policy explicitly, the key will be non-exportable.

Fixes #5076
Fixes #9159

[learn-build-service-prod]

Learn Build status updates of commit d7eb604:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Security.Cryptography/CngExportPolicies.xml	✅Succeeded	View

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[BillWagner]

This looks right to me, but adding @bartonjs to confirm.

[joynoele]

bumping this PR.