Review asked for this directly: the hardware layout above port_l2mc_set()
would be better as documentation than as a comment, keeping only the two
lines that say what the function does.
doc/l2.md gains a section on static multicast entries, why delivery uses
the forward action rather than the trap, and the SMI layout of the entry.
doc/CpuPort.md gains the layout of the tag's flags and pmask words, with
the byte order trap that cost an afternoon: writing the flags constant raw
instead of through HTONS puts 0x0020 on the wire as 0x2000, which is EFID
rather than LEARN_DIS, and the ASIC then leaves the 0x8899 header on the
frame.
The comments those paragraphs came from are replaced by a pointer to the
file that now holds them.