Embedded Symbols for NuGet documentation is misleading

In the snippet of code with `` including *.pdb, that works for embedded symbols. However, that appears to point the symbols back to the location on disk at the time that the NuGet package was generated. This will only work for local development, and therefore not for the 99% of scenarios.

Repro steps:

1. Create a simple class library NuGet package.
2. Generate the .nupkg file using `$(AllowedOutputExtensionsInPackageBuildOutputFolder);.pdb` from the documentation.
3. Create a simple console application and reference the NuGet package created in step 2.
4. Debug the console application, and step into a method call from the NuGet package created in step 2.
5. Observe that the source code is loaded appropriately.
6. Delete the project created in step 1 from disk. (do not alter the console app from step 3, or the .nupkg generated in step 2)
7. Repeat step 4.
8. Observe that the source code is no longer loaded appropriately. Instead, the IDE will decompile on the fly.

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* ID: fc2b4abb-c382-17ef-f1f3-1413ee27aaa0
* Version Independent ID: 4d4bb4d2-a8f4-f142-7154-40ce7fc7ae32
* Content: [NuGet and .NET libraries](https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/nuget?msclkid=fa776a88d15811ec900c35551e2601e7)
* Content Source: [docs/standard/library-guidance/nuget.md](https://github.com/dotnet/docs/blob/main/docs/standard/library-guidance/nuget.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**