docs: netconsole: document local_ip auto-selection

An unset local_ip is auto-populated from the egress interface at enable
time, but any value the user writes, including 0.0.0.0, is used as-is.
This deviates from the behavior prior to "netconsole: use the address
family instead of the ipv6 flag", which interpreted 0.0.0.0 as unset.

Document this so the distinction between "unset" and "set to 0.0.0.0" is
clear.

Reviewed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Gustavo Luiz Duarte <gustavold@gmail.com>
Link: https://patch.msgid.link/20260903-netcons_ipv6-v4-7-bdd183c844d3@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>