Documentation should include whether the type is part of platform or ships out-of-band via nuget.org

Consider these two classes:
```
SignedCms Class
Namespace: System.Security.Cryptography.Pkcs
Assembly: System.Security.Cryptography.Pkcs.dll
.NET: 5.0
.NET Core: 3.1 3.0
.NET Platform Extensions: 5.0 3.1 3.0 2.2 2.1
```

```
Process Class
Namespace: System.Diagnostics
Assembly: System.Diagnostics.Process.dll
Applies to
.NET: 5.0
.NET Core: 3.1 3.0 2.2 2.1 2.0 1.1 1.0
.NET Standard: 2.1 2.0
```

It's not clear from the docs `SignedCms` class requires a reference to the [System.Security.Cryptography.Pkcs](https://www.nuget.org/packages/System.Security.Cryptography.Pkcs/) nuget.org package,
while `Process` ships as part of the .NET 5 framework.