Incomplete DefaultDllImportSearchPaths docs

https://github.com/dotnet/dotnet-api-docs/blob/8adb892bc4a3f45fdeaa24f716d2dbd4ba0365ab/xml/System.Runtime.InteropServices/DefaultDllImportSearchPathsAttribute.xml

.NET Core 3.0 will bring `DefaultDllImportSearchPathsAttribute`, and [the Roslyn analyzers will flag P/Invokes that lack `DefaultDllImportSearchPathsAttribute` by default](https://github.com/dotnet/roslyn-analyzers/pull/2658). However, the documentation on `DefaultDllImportSearchPathsAttribute` is Windows-centric; it doesn't describe how `DefaultDllImportSearchPathsAttribute` works on Linux, for example.

This is a problem for code such at the following snippet taken from https://github.com/dotnet/corefx/blob/master/src/Common/src/Interop/Unix/System.Security.Cryptography.Native/Interop.RAND.cs (with a constant replaced with a literal):

``` csharp
[DllImport("System.Security.Cryptography.Native.OpenSsl")]
[return: MarshalAs(UnmanagedType.Bool)]
private static extern unsafe bool CryptoNative_GetRandomBytes(byte* buffer, int length);
```

The options available for `DefaultDllImportSearchPathsAttribute` are aimed at Windows, and the documentation doesn't make it clear how to handle this. Is the attribute completely ignored? Are only certain options available?