Add a comment to refer to the section of the programmer's manual that explains what the pointer tagging in Use is for.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@70521 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Use.h

@@ -11,6 +11,15 @@
 // instruction or some other User instance which refers to a Value.  The Use
 // class keeps the "use list" of the referenced value up to date.
 //
+// Pointer tagging is used to efficiently find the User corresponding
+// to a Use without having to store a User pointer in every Use. A
+// User is preceded in memory by all the Uses corresponding to its
+// operands, and the low bits of one of the fields (Prev) of the Use
+// class are used to encode offsets to be able to find that User given
+// a pointer to any Use. For details, see:
+//
+//   http://www.llvm.org/docs/ProgrammersManual.html#UserLayout
+//
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_USE_H