Remove Sendable conformance from FileDescriptor, add comment

Sources/System/FileDescriptor.swift

@@ -475,8 +475,13 @@ extension FileDescriptor.OpenOptions
 }
 
 #if compiler(>=5.5) && canImport(_Concurrency)
-@available(*, unavailable, message: "FileDescriptors aren't safe to use across multiple threads.")
-extension FileDescriptor: Sendable {}
+// File descriptors aren't necessarily safe to use across threads.
+// Thus no explicit `Sendable` conformance.
+// They can be used in a safe way, however,
+// which is why the conformance cannot be marked with `@available(*, unavailable)`.
+
+// extension FileDescriptor: Sendable {}
+
 extension FileDescriptor.AccessMode: Sendable {}
 extension FileDescriptor.OpenOptions: Sendable {}
 extension FileDescriptor.SeekOrigin: Sendable {}