doc: fix wrong arch/i386 references

Change all "arch/i386" to "arch/x86" in Documentaion/,
since the directory has changed.

Also update the files which have changed their filename
in the meantime accordingly.

Signed-off-by: Wanlong Gao <wanlong.gao@gmail.com>
[jkosina@suse.cz: reword changelog]
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/scheduler/sched-arch.txt b/Documentation/scheduler/sched-arch.txt
index d43dbcb..28aa107 100644
--- a/Documentation/scheduler/sched-arch.txt
+++ b/Documentation/scheduler/sched-arch.txt
@@ -66,7 +66,7 @@
 	    barrier issued (followed by a test of need_resched with
 	    interrupts disabled, as explained in 3).
 
-arch/i386/kernel/process.c has examples of both polling and
+arch/x86/kernel/process.c has examples of both polling and
 sleeping idle functions.