Add link to intel instruction set reference

Author: mtak-

crates/core_arch/src/x86/rtm.rs

@@ -2,10 +2,14 @@
 //! 
 //! This CPU feature is available on Intel Broadwell or later CPUs (and some Haswell).
 //! 
+//! The reference is [Intel 64 and IA-32 Architectures Software Developer's
+//! Manual Volume 2: Instruction Set Reference, A-Z][intel64_ref].
+//! 
 //! [Uncyclopedia][wikipedia_rtm] provides a quick overview of the assembly instructions, and
 //! Intel's [programming considerations][intel_consid] details what sorts of instructions within a
 //! transaction are likely to cause an abort.
 //! 
+//! [intel64_ref]: http://www.intel.de/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf
 //! [wikipedia_rtm]: https://en.wikipedia.org/wiki/Transactional_Synchronization_Extensions#Restricted_Transactional_Memory
 //! [intel_consid]: https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-intel-transactional-synchronization-extensions-intel-tsx-programming-considerations