Document source and versioning of the TCPInfo struct.

PiperOrigin-RevId: 263637194

pkg/abi/linux/socket.go

@@ -292,7 +292,10 @@ const SizeOfLinger = 8
 
 // TCPInfo is a collection of TCP statistics.
 //
-// From uapi/linux/tcp.h.
+// From uapi/linux/tcp.h. Newer versions of Linux continue to add new fields to
+// the end of this struct or within existing unusued space, so its size grows
+// over time. The current iteration is based on linux v4.17. New versions are
+// always backwards compatible.
 type TCPInfo struct {
 	State       uint8
 	CaState     uint8