Add comment about error code

Author: AZero13

Tests/FoundationEssentialsTests/FileManager/FileManagerTests.swift

@@ -1071,6 +1071,7 @@ private struct FileManagerTests {
                 if error.code == .featureUnsupported { return }
                 guard let posix = error.underlying as? POSIXError else { throw error }
                 #if canImport(Darwin)
+                // .ENOTSUP is Darwin-specific.
                 guard posix.code == .ENOTSUP || posix.code == .EOPNOTSUPP else { throw error }
                 #else
                 guard posix.code == .EOPNOTSUPP else { throw error }