add reference.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@12794 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ia64.s

@@ -10,6 +10,10 @@
 // It's because BSP is updated by br.call/brl.call (not alloc instruction).
 // So rb_ia64_flushrs flushes stack frames including caller's one.
 // rb_ia64_bsp returns the address next to caller's register stack frame. 
+//
+// See also
+// Intel Itanium Architecture Software Developer's Manual
+// Volume 2: System Architecture.
 //
 	.file	"ia64.c"
 	.text