Documentation: netconsole: add support for cmdline targets

Documentation: netconsole: add support for cmdline targets

With the previous patches, there is no more limitation at modifying the
targets created at boot time (or module load time).

Document the way on how to create the configfs directories to be able to
modify these netconsole targets.

The design discussion about this topic could be found at:
https://lore.kernel.org/all/ZRWRal5bW93px4km@gmail.com/

Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20231012111401.333798-5-leitao@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>