General updates. #17951, #17020, and #9884

[IEvangelist]

Summary

• Added clarification around <NetStandardImplicitPackageVersion> usage Meaning of <NetStandardImplicitPackageVersion> element #17951
• Verified that the .NET Core section properly details references, as suggested in Suggested improvements #9884
• I'm intrigued by Relevance of packages, metapackages, and frameworks article. #17020, as I believe this is still relevant - the metapackages still exist and are still used (this article simply details them):
  • https://www.nuget.org/packages/Microsoft.NETCore.App
  • https://www.nuget.org/packages/Microsoft.AspNETCore.App
  • https://www.nuget.org/packages/Microsoft.AspNETCore.All
  • https://www.nuget.org/packages/Microsoft.NETCore.Portable.Compatibility

Finally, after searching through all of the existing samples, tutorials, and docs I have been unable to find any incorrect references to metapackage usage, or legacy netcoreapp1.x usages. I believe that this also verifies #2726. All of the files where the term "metapackage" is used are accurate.

• .\docs\core\packages.md
• .\docs\core\toc.yml
• .\docs\core\deploying\index.md
• .\docs\core\porting\libraries.md
• .\docs\core\project-sdk\msbuild-props.md
• .\docs\core\tools\csproj.md
• .\docs\standard\frameworks.md
• .\docs\standard\glossary.md
• .\docs\standard\assembly\reference-assemblies.md

Fixes #2726, closes #9884, resolves #17020, fixes #17951

[BillWagner]

This LGTM @IEvangelist

You can when ready.