Add a clarifying comment to the memory layout navigation. (Inspired by Coverity CID 100022)

courgette/memory_allocator.cc

Index: courgette/memory_allocator.cc
diff --git a/courgette/memory_allocator.cc b/courgette/memory_allocator.cc
index f2aed22d589296a8f983f062f95c99b0009bdd40..8f041061bc9d59c4a65f34785db3ebe673a5b49b 100644
--- a/courgette/memory_allocator.cc
+++ b/courgette/memory_allocator.cc
@@ -135,6 +135,8 @@ bool TempMapping::Initialize(size_t size) {
 
 void* TempMapping::memory() const {
   uint8* mem = reinterpret_cast<uint8*>(mapping_.view());
+  // The 'this' pointer is written at the start of mapping_.view(), so
+  // go past it. (See Initialize()).
   if (mem)
     mem += sizeof(this);
   DCHECK(mem);