[LangRef] Add link to opaque pointers

[aeubanks]

thanks!

llvm/docs/LangRef.rst

 Pointer types may have an optional address space attribute defining the
 numbered address space where the pointed-to object resides. The default
 address space is number zero. The semantics of non-zero address spaces
 are target-specific.
 
 Note that LLVM does not permit pointers to void (``void*``) nor does it
 permit pointers to labels (``label*``). Use ``i8*`` instead.
 
-LLVM is in the process of transitioning to opaque pointers. Opaque pointers do
-not have a pointee type. Rather, instructions interacting through pointers
-specify the type of the underlying memory they are interacting with. Opaque
-pointers are still in the process of being worked on and are not complete.
+LLVM is in the process of transitioning to
+`opaque pointers <OpaquePointers.html#opaque-pointers>`_.
+Opaque pointers do not have a pointee type. Rather, instructions
+interacting through pointers specify the type of the underlying memory
+they are interacting with. Opaque pointers are still in the process of
+being worked on and are not complete.
 
 :Syntax:
 
 ::
 
       <type> *
       ptr