Documentation/serial-console: Document the behavior when the last console= parameter...

Documentation/serial-console: Document the behavior when the last console= parameter is not used

The console= kernel command-line parameter defines where the kernel
messages appear. It can be used multiple times to make the kernel log
visible on more devices.

The ordering of the console= parameters is important. In particular,
the last one defines which device can be accessed also via /dev/console.

The behavior is more complicated when the last console= parameter is
ignored by kernel. It might be surprising because it was not intentional.
The kernel just works this way historically.

There were few attempts to change the behavior. Unfortunately, it can't
be done because it would break existing users. Document the historical
behavior at least.

Link: https://lore.kernel.org/r/20170606143149.GB7604@pathway.suse.cz
Link: https://lore.kernel.org/r/20230213113912.1237943-1-rkanwal@rivosinc.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230308112433.24292-1-pmladek@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>