Add comments.

src/tiffcomposite_int.hpp

@@ -436,7 +436,21 @@ namespace Exiv2 {
         void encode(TiffEncoder& encoder, const Exifdatum* datum);
         //! Set the offset
         void setOffset(int32_t offset) { offset_ = offset; }
-        //! Set pointer and size of the entry's data (not taking ownership of the data).
+        /*!
+          @brief Set pointer and size of the entry's data (not taking ownership of the data).
+
+          @param storage Usually, pData is a pointer into a copy of the image file, which
+                         means that it points to memory which is guaranteed to live longer
+                         than this class. However, sometimes pData is pointer into a
+                         DataBuf that was allocated by another node in the component tree.
+                         If so, we need to make sure that the DataBuf doesn't get freed too
+                         early. We use a std::shared_ptr to hold a reference to the DataBuf
+                         to ensure that it will be kept alive. The storage parameter is
+                         assigned to the storage_ field. In the more common scenario where
+                         pData points to a copy of the image, rather than a DataBuf, then
+                         you should pass std::shared_ptr<DataBuf>(), which is essentially
+                         a nullptr.
+         */
         void setData(byte* pData, int32_t size, const std::shared_ptr<DataBuf>& storage);
         /*!
           @brief Set the entry's data buffer. A shared_ptr is used to manage the DataBuf
@@ -533,6 +547,7 @@ namespace Exiv2 {
                                     TiffType  tiffType,
                                     ByteOrder byteOrder);
 
+        //! Used (internally) to create another reference to the DataBuf reference by storage_.
         const std::shared_ptr<DataBuf>& storage() { return storage_; }
 
     private: