AArch64: add block comments where missing

Only comments affected. No code change at all.

llvm-svn: 175169
diff --git a/llvm/lib/Target/AArch64/Disassembler/AArch64Disassembler.cpp b/llvm/lib/Target/AArch64/Disassembler/AArch64Disassembler.cpp
index 38d0e8e..eba7666 100644
--- a/llvm/lib/Target/AArch64/Disassembler/AArch64Disassembler.cpp
+++ b/llvm/lib/Target/AArch64/Disassembler/AArch64Disassembler.cpp
@@ -1,4 +1,4 @@
-//===- AArch64Disassembler.cpp - Disassembler for AArch64/Thumb ISA -------===//
+//===- AArch64Disassembler.cpp - Disassembler for AArch64 ISA -------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
@@ -6,6 +6,12 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+//
+// This file contains the functions necessary to decode AArch64 instruction
+// bitpatterns into MCInsts (with the help of TableGenerated information from
+// the instruction definitions).
+//
+//===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "arm-disassembler"