Docs: printk: add 'console=null|""' to admin/kernel-parameters

Tell about 'console=null|""' and how to use it.

It can be helpful to set (enable) CONFIG_NULL_TTY so that the ttynull
driver is available. This avoids problems with stdin/stdout/stderr of
the init process. Howevere, CONFIG_NULL_TTY cannot be enabled by default
because it can be used by mistake, see the commit a91bd62 ("Revert
"init/console: Use ttynull as a fallback when there is no console"").

Signed-off-by: Randy Dunlap <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: John Ogness <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: [email protected]
[[email protected]: Slightly update wording.]
Signed-off-by: Petr Mladek <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: rddunlap

Documentation/admin-guide/kernel-parameters.txt

@@ -724,6 +724,12 @@
 		hvc<n>	Use the hypervisor console device <n>. This is for
 			both Xen and PowerPC hypervisors.
 
+		{ null | "" }
+			Use to disable console output, i.e., to have kernel
+			console messages discarded.
+			This must be the only console= parameter used on the
+			kernel command line.
+
 		If the device connected to the port is not a TTY but a braille
 		device, prepend "brl," before the device type, for instance
 			console=brl,ttyS0