i3-companion: document polybar wire-format

bin/i3-companion

@@ -265,7 +265,11 @@ def polybar(module):
 
             # Send it to polybar
             cmd = bytes(f"#{module}.send.{content}", "utf-8")
-            data = b"polyipc" + struct.pack("=BIB", 0, len(cmd), 2) + cmd
+            data = (
+                b"polyipc"  # magic
+                + struct.pack("=BIB", 0, len(cmd), 2)  # header: version, length, type
+                + cmd
+            )
             for name in glob.glob(f"{os.getenv('XDG_RUNTIME_DIR')}/polybar/*.sock"):
                 try:
                     reader, writer = await asyncio.open_unix_connection(name)