docs: netconsole: document release feature

Add documentation explaining the kernel release auto-population feature
in netconsole.

This feature appends kernel version information to the userdata
dictionary in every message sent when enabled via the `release_enabled`
file in the configfs hierarchy.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250314-netcons_release-v1-6-07979c4b86af@debian.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>