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

Documentation/admin-guide/kernel-parameters.txt

@@ -4777,7 +4777,7 @@
 
 	reboot=		[KNL]
 			Format (x86 or x86_64):
-				[w[arm] | c[old] | h[ard] | s[oft] | g[pio]] \
+				[w[arm] | c[old] | h[ard] | s[oft] | g[pio]] | d[efault] \
 				[[,]s[mp]#### \
 				[[,]b[ios] | a[cpi] | k[bd] | t[riple] | e[fi] | p[ci]] \
 				[[,]f[orce]

Documentation/x86/x86_64/boot-options.rst

@@ -157,6 +157,13 @@ Rebooting
      Don't stop other CPUs on reboot. This can make reboot more reliable
      in some cases.
 
+   reboot=default
+     There are some built-in platform specific "quirks" - you may see:
+     "reboot: <name> series board detected. Selecting <type> for reboots."
+     In the case where you think the quirk is in error (e.g. you have
+     newer BIOS, or newer board) using this option will ignore the built-in
+     quirk table, and use the generic default reboot actions.
+
 Non Executable Mappings
 =======================