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/[e-mail]/

Signed-off-by: Breno Leitao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/netconsole.rst

···
 Dynamic reconfigurability is a useful addition to netconsole that enables
 remote logging targets to be dynamically added, removed, or have their
 parameters reconfigured at runtime from a configfs-based userspace interface.
-[ Note that the parameters of netconsole targets that were specified/created
-from the boot/module option are not exposed via this interface, and hence
-cannot be modified dynamically. ]
 
 To include this feature, select CONFIG_NETCONSOLE_DYNAMIC when building the
 netconsole module (or kernel, if netconsole is built-in).
···
 You can also update the local interface dynamically. This is especially
 useful if you want to use interfaces that have newly come up (and may not
 have existed when netconsole was loaded / initialized).
+
+Netconsole targets defined at boot time (or module load time) with the
+`netconsole=` param are assigned the name `cmdline<index>`.  For example, the
+first target in the parameter is named `cmdline0`.  You can control and modify
+these targets by creating configfs directories with the matching name.
+
+Let's suppose you have two netconsole targets defined at boot time::
+
+ netconsole=4444@10.0.0.1/eth1,9353@10.0.0.2/12:34:56:78:9a:bc;4444@10.0.0.1/eth1,9353@10.0.0.3/12:34:56:78:9a:bc
+
+You can modify these targets in runtime by creating the following targets::
+
+ mkdir cmdline0
+ cat cmdline0/remote_ip
+ 10.0.0.2
+
+ mkdir cmdline1
+ cat cmdline1/remote_ip
+ 10.0.0.3
 
 Extended console:
 =================