printk: Document that CONFIG_BOOT_PRINTK_DELAY required for boot_delay=

Document the fact that CONFIG_BOOT_PRINTK_DELAY must be enabled for the
"boot_delay" kernel parameter to work.  Also mention that "lpj=" may be
necessary.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20230126225420.1320276-1-helgaas@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>