Note ImmArg in documentation for adding intrinsics

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356145 91177308-0d34-0410-b5e6-96231b3b80d8

docs/ExtendingLLVM.rst

@@ -47,11 +47,13 @@ function and then be turned into an instruction if warranted.
 
 #. ``llvm/include/llvm/IR/Intrinsics*.td``:
 
-   Add an entry for your intrinsic.  Describe its memory access characteristics
-   for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note
-   that any intrinsic using one of the ``llvm_any*_ty`` types for an argument or
-   return type will be deemed by ``tblgen`` as overloaded and the corresponding
-   suffix will be required on the intrinsic's name.
+   Add an entry for your intrinsic.  Describe its memory access
+   characteristics for optimization (this controls whether it will be
+   DCE'd, CSE'd, etc). If any arguments need to be immediates, these
+   must be indicated with the ImmArg property. Note that any intrinsic
+   using one of the ``llvm_any*_ty`` types for an argument or return
+   type will be deemed by ``tblgen`` as overloaded and the
+   corresponding suffix will be required on the intrinsic's name.
 
 #. ``llvm/lib/Analysis/ConstantFolding.cpp``: