Mark FileDescriptor Sendable conformance as unavailable

Sources/System/FileDescriptor.swift

@@ -476,10 +476,11 @@ extension FileDescriptor.OpenOptions
 }
 
 #if compiler(>=5.5) && canImport(_Concurrency)
-// File descriptors aren't necessarily safe to use across threads.
-// However, since they can be used in a safe way,
-// we do make them `Sendable` to not make it unnecessarily complicated to
-// use them across concurrency boundaries in a safe way.
+// Since some file descriptor operations aren't safe to use across threads,
+// we mark the Sendable conformance as unavailble.
+// To use file descriptors with operations that *are* thread-safe,
+// wrap them in an `@unchecked Sendable` type.
+@available(*, unavailable, message: "File descriptors are not completely thread-safe.")
 extension FileDescriptor: Sendable {}
 
 @available(/*System 0.0.1: macOS 11.0, iOS 14.0, watchOS 7.0, tvOS 14.0*/iOS 8, *)