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: WANG Cong <amwang@redhat.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>