Doc: move Documentation/exception.txt into x86 subdir

exception.txt only explains the code on x86, so it's better to
move it into Documentation/x86 directory.

And also rename it to exception-tables.txt which looks much
more reasonable.

This patch is on top of the previous one.
Signed-off-by: NWANG Cong <amwang@redhat.com>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/x86/00-INDEX

@@ -2,3 +2,5 @@
 	- this file
 mtrr.txt
 	- how to use x86 Memory Type Range Registers to increase performance
+exception-tables.txt
+	- why and how Linux kernel uses exception tables on x86