Improvements to TableGen/LangIntro.rst

Document the 'code' data type, and that value{15-17} is different to 
value{17-15}.

Patch by @chenwj (Wei-Ren Chen).

Differential Revision: https://reviews.llvm.org/D32117



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301920 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed