Port System.Formats.Asn1 docs for .NET 9

[vcsjones]

A new API was introduced in .NET 9, {Try}DecodeLength ref. It's not clear to me why this was not flagged as a package that needed to be updated, but we never ported the docs for this as far as I can tell.

Regardless, this ports the documentation for System.Formats.Asn1.

/cc @bartonjs @carlossanlop

[gewarren]

@vcsjones If this was set to true, the docs would have been pulled in automatically during the CI updates: https://github.com/dotnet/runtime/blob/def5e3240bdee3ee37ba22c41c840bbf431c4b15/src/libraries/System.Formats.Asn1/src/System.Formats.Asn1.csproj#L7

[learn-build-service-prod]

Learn Build status updates of commit 6729d89:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Formats.Asn1/AsnDecoder.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod]

Learn Build status updates of commit 3f3b0fd:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Formats.Asn1/AsnDecoder.xml	✅Succeeded	View

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel