x86/reboot: Document how to override DMI platform quirks

commit 5955633e ("x86/reboot: Skip DMI checks if reboot set by user")
made it so that it's not required to recompile the kernel in order to
bypass broken reboot quirks compiled into an image:

 * This variable is used privately to keep track of whether or not
 * reboot_type is still set to its default value (i.e., reboot= hasn't
 * been set on the command line).  This is needed so that we can
 * suppress DMI scanning for reboot quirks.  Without it, it's
 * impossible to override a faulty reboot quirk without recompiling.

However, at the time it was not eally documented outside the source code,
and so this information isn't really available to the average user out
there.

The change is a little white lie and invented "reboot=default" since it is
easy to remember, and documents well.  The truth is that any random string
that is *not* a currently accepted string will work.

Since that doesn't document well for non-coders, and since it's unknown
what the future additions might be, lay claim on "default" since that is
exactly what it achieves.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210530162447.996461-3-paul.gortmaker@windriver.com