Add note that DOTNET_ADDITIONAL_DEPS only works with framework-dependent applications

[Copilot]

The DOTNET_ADDITIONAL_DEPS environment variable is a key mechanism for runtime store activation in hosting startup assemblies, but it has an important limitation that wasn't documented: it only works with framework-dependent applications and is ignored for self-contained applications.

This PR adds clear documentation of this limitation by adding NOTE blocks in the Runtime Store section immediately after DOTNET_ADDITIONAL_DEPS is introduced. The notes are added in both moniker ranges (>=aspnetcore-3.0 and <aspnetcore-3.0) to ensure all users are informed of this behavior.

Changes made:

• Added identical NOTE blocks explaining the framework-dependent application requirement
• Updated the ms.date metadata to reflect the documentation update
• Used proper Microsoft Learn NOTE syntax for consistency with existing documentation

This documentation enhancement will help prevent confusion when developers attempt to use runtime stores with self-contained applications, as they'll now understand why the DOTNET_ADDITIONAL_DEPS environment variable has no effect in that deployment model.

Fixes #35935.

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aspnetcore/fundamentals/host/platform-specific-configuration.md	Use hosting startup assemblies in ASP.NET Core

[wadepickett]

Looks great. Just a minor correction to the ms.date you could take or leave.