Clarify IsTrimmable assembly metadata

[sbomer]

The trimming options https://docs.microsoft.com/dotnet/core/deploying/trimming/trimming-options?pivots=dotnet-7-0 should give more clarity on what [AssemblyMetadata("IsTrimmable", "True")] means (and point out that [AssemblyMetadata("IsTrimmable", "False")] is not supported.

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• ID: 811fc2ce-1f05-5be2-d033-7fce35e49525
• Version Independent ID: fca1c728-4716-8c28-89e1-02eb947bf602
• Content: Trimming options - .NET
• Content Source: docs/core/deploying/trimming/trimming-options.md
• Product: dotnet-fundamentals
• GitHub Login: @sbomer
• Microsoft Alias: svbomer

[sbomer]

Looking over the mentions of this in the trimming options doc, I think the current docs are mostly sufficient. In most cases, we want developers to use the high-level options <PublishTrimmed> and <IsTrimmable> that add the assembly metadata.

We have changed the defaults to trim all assemblies, so this is primarily relevant to partial TrimMode. I don't think we should even mention the unsupported False case, as doing so would probably cause more confusion.

Making a small clarification in #42317.