Improve documentation about StringFactory.newStringFromChars.

Make it clear that the native method requires its third
argument to be non-null, and therefore that the intrinsics
do not need a null check for it.

Bug: 27378573
Change-Id: Id2f78ceb0f7674f1066bc3f216b738358ca25542
diff --git a/compiler/optimizing/intrinsics_x86_64.cc b/compiler/optimizing/intrinsics_x86_64.cc
index 2a9e684..15c3997 100644
--- a/compiler/optimizing/intrinsics_x86_64.cc
+++ b/compiler/optimizing/intrinsics_x86_64.cc
@@ -1659,6 +1659,12 @@
 void IntrinsicCodeGeneratorX86_64::VisitStringNewStringFromChars(HInvoke* invoke) {
   X86_64Assembler* assembler = GetAssembler();
 
+  // No need to emit code checking whether `locations->InAt(2)` is a null
+  // pointer, as callers of the native method
+  //
+  //   java.lang.StringFactory.newStringFromChars(int offset, int charCount, char[] data)
+  //
+  // all include a null check on `data` before calling that method.
   __ gs()->call(Address::Absolute(QUICK_ENTRYPOINT_OFFSET(kX86_64WordSize, pAllocStringFromChars),
                                   /* no_rip */ true));
   codegen_->RecordPcInfo(invoke, invoke->GetDexPc());