doc: Update netconsole examples, mention stderr

Stderr was missing from the initial description and example.

As I understand the env command documentation the subcommand style is
preferred, though the old format is still fully supported.

Signed-off-by: Fiona Klute <fiona.klute@gmx.de>
Reviewed-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>