[PATCH] x86_64: Document backtracer selection options

Signed-off-by: Andi Kleen <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/x86_64/boot-options.txt

···
   pagefaulttrace Dump all page faults. Only useful for extreme debugging
 		and will create a lot of output.
 
+  call_trace=[old|both|newfallback|new]
+		old: use old inexact backtracer
+		new: use new exact dwarf2 unwinder
+ 		both: print entries from both
+		newfallback: use new unwinder but fall back to old if it gets
+			stuck (default)
+
 Misc
 
   noreplacement  Don't replace instructions with more appropriate ones