[AMDGPU][MC][DOC] Updated AMD GPU assembler description

Stage 2: added detailed description of operands

See bug 36572: https://bugs.llvm.org/show_bug.cgi?id=36572

llvm-svn: 349368
GitOrigin-RevId: 47eb63684d22c12b9defb99c394dbd10e26fefff
diff --git a/docs/AMDGPU/gfx9_opt.rst b/docs/AMDGPU/gfx9_opt.rst
new file mode 100644
index 0000000..50d73f9
--- /dev/null
+++ b/docs/AMDGPU/gfx9_opt.rst
@@ -0,0 +1,14 @@
+..
+    **************************************************
+    *                                                *
+    *   Automatically generated file, do not edit!   *
+    *                                                *
+    **************************************************
+
+.. _amdgpu_synid9_opt:
+
+opt
+===========================
+
+This is an optional operand. It must be used if and only if :ref:`glc<amdgpu_synid_glc>` is specified.
+