Added comment warning that the file size attribute only deals with the data fork on the Mac and point to Mac specific interface for dealing with the size of the combined forks. r=none as this is just cleanup.

xpcom/io/nsIFile.idl

@@ -170,6 +170,12 @@ interface nsIFile : nsISupports
     attribute PRInt64 lastModificationDate;
     attribute PRInt64 lastModificationDateOfLink;
     
+    /**
+     * WARNING!  On the Mac getting/setting the file size with nsIFile
+     * only deals with the size of the data fork.  If you need to
+     * know the size of the combined data and resource forks use the
+     * GetFileSizeWithResFork() method defined in nsILocalFileMac.h
+     */
     attribute PRInt64 fileSize;
     readonly attribute PRInt64 fileSizeOfLink;