Re-add Sendable conformance to FileDescriptor, adjust comment

Sources/System/FileDescriptor.swift

@@ -476,12 +476,10 @@ extension FileDescriptor.OpenOptions
 
 #if compiler(>=5.5) && canImport(_Concurrency)
 // 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 {}
-
+// 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.
+extension FileDescriptor: Sendable {}
 extension FileDescriptor.AccessMode: Sendable {}
 extension FileDescriptor.OpenOptions: Sendable {}
 extension FileDescriptor.SeekOrigin: Sendable {}