Incomplete DefaultDllImportSearchPaths docs

[Zenexer]

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. 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):

[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?

[Zenexer]

Related: dotnet/roslyn-analyzers#2855

[mairaw]

@AaronRobinsonMSFT, @jkoritzinsky can you guys help with this feedback? Is there anything we can add to the docs about this API behavior in .NET Core? Thanks.

[jkoritzinsky]

I've just checked to confirm, and the DllImportSearchPath enumeration values are ignored on non-Windows. We should add that to the docs.