[X86] Add SERIALIZE instruction.

Summary: For more details about this instruction, please refer to the latest ISE document: https://software.intel.com/en-us/download/intel-architecture-instruction-set-extensions-programming-reference

Reviewers: craig.topper, RKSimon, LuoYuanke

Reviewed By: craig.topper

Subscribers: mgorny, hiraditya, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77193
diff --git a/clang/docs/ClangCommandLineReference.rst b/clang/docs/ClangCommandLineReference.rst
index 3f0e471..51d4f32 100644
--- a/clang/docs/ClangCommandLineReference.rst
+++ b/clang/docs/ClangCommandLineReference.rst
@@ -3145,6 +3145,8 @@
 
 .. option:: -msahf, -mno-sahf
 
+.. option:: -mserialize, -mno-serialize
+
 .. option:: -msgx, -mno-sgx
 
 .. option:: -msha, -mno-sha