doc: add comment for iface name patch in fixnemu

demo/fixnemu.py

@@ -75,7 +75,10 @@ get_addr_data.func_code = _get_addr_data.func_code
 @staticmethod
 def _gen_if_name():
     n = Interface._gen_next_id()
-    # Max 15 chars
+    # The maximum size of the interface name is 15(char [IFNAMSIZ(16)])
+    # len(NETNSbr-) = 8, So we need to limit the part "%.4x%.3x" % (os.getpid(), n)
+    # to size 7. That means PID < 0xffff, n < 0xfff
+    # PID often exceeds 0xffff, so use mod 0xffff to limit it length.
     return "NETNSif-%.4x%.3x" % (os.getpid() % 0xffff, n)
 Interface._gen_if_name = _gen_if_name