docs: correct x86 MCE command line option info

Not even the types were correct, let alone defaults being spelled out or
the purpose of the options actually mentioned in any way.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

docs/misc/xen-command-line.pandoc

@@ -1691,10 +1691,23 @@ one pending bit to be allocated.
 Defaults to 20 bits (to cover at most 1048576 interrupts).
 
 ### mce (x86)
-> `= <integer>`
+> `= <boolean>`
+
+> Default: `true`
+
+Allows to disable the use of Machine Check Exceptions.  Note that doing
+so may result in silent shutdown of the system in case an event occurs
+which would have resulted in raising a Machine Check Exception.  Silent
+here is as far as Xen is concerned; firmware may offer to retrieve some
+collected data.
 
 ### mce_fb (Intel)
-> `= <integer>`
+> `= <boolean>`
+
+> Default: `false`
+
+Force broadcasting of Machine Check Exceptions, suppressing the use of
+Local MCE functionality available in newer Intel hardware.
 
 ### mce_verbosity (x86)
 > `= verbose`