`PackageIconFullPath` isn't mentioned in library guidance

@JamesNK, Does `PackageIconFullPath` need to be documented in this article as well? with an explanation whether it's needed together with `PackageIcon` or not?

_Originally posted by @Youssef1313 in https://github.com/dotnet/docs/pull/19628#discussion_r462563196_

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#### Document Details

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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#feedback)
* Content Source: [docs/standard/library-guidance/nuget.md](https://github.com/dotnet/docs/blob/master/docs/standard/library-guidance/nuget.md)
* Product: **dotnet**
* GitHub Login: @JamesNK
* Microsoft Alias: **jamesnk**