PR micropython#10635 introduced this limit which seems rather arbitrary and wildly
insufficient to me.

A 16 bytes length limits means a 15 character string limit, which even
`ubinascii.hexlify(machine.unique_id())` on most ports will break. And
then assuming you just supply a random 15 byte hex string, most people
won't be able to identify the hosts function in a list on their wifi
routers.

I wanted to set my hostname to `rpi-fan-controller-{hex_uid}` and just
got a ValueError without explanation. It took me 2 hours to figure what
went wrong. Because of this I added a note about the exception and
limitation to the documentation.

Since I didn't really understand the reason for 16 bytes I set the new
limit to the size used in all involved protocols: DHCP 64 bytes, DNS 63
"octets" (characters) plus length byte, and mDNS which points to the DNS
RFC. I can imagine that some ports for small boards might want to
decrease the size of this statically allocated buffer, and from what I
gathered from the C code it would be a simple as defining
`MICROPY_PY_NETWORK_HOSTNAME_MAX_LEN`, so I mentioned this possibility
for end users in the documentation as well.

Signed-off-by: Simon Lutz Brüggen <micropython@m3t0r.de>