lacp: keep comments to the frame layout

The comments repeated the documentation, described tests or explained
the code to its author. What stays describes the LACPDU structures. The
timer constants are decimal, in units of the LACP timer.

doc/CpuPort.md

@@ -79,11 +79,6 @@ bit7  KEEP    | 6 VSEL     | 5 LEARN_DIS         | 4:0 VIDX
 
 All fields are in network byte order.
 
-This word has to be written through `HTONS` like every other field of the tag.
-Writing a constant raw puts the bits in the wrong byte, so `0x0020` reaches the
-wire as `0x2000`, which is EFID rather than LEARN_DIS. The ASIC then fails to
-parse the tag and forwards the frame with the `0x8899` header still on it.
-
 * `EFID_EN`, `EFID`: look the destination up under this filtering ID
   instead of the port's own
 * `PRI_EN`, `PRI`: force the given priority on the frame