Add comment on tinygltf::ExpandFilePath()

tiny_gltf.h

diff --git a/tiny_gltf.h b/tiny_gltf.h
index 708ca02..fa16ec5 100644
--- a/tiny_gltf.h
+++ b/tiny_gltf.h
@@ -1249,7 +1249,13 @@
 
 bool FileExists(const std::string &abs_filename, void *);
 
-std::string ExpandFilePath(const std::string &filepath, void *);
+///
+/// Expand file path(e.g. `~` to home directory on posix, `%APPDATA%` to `C:\Users\tinygltf\AppData`)
+///
+/// @param[in] filepath File path string. Assume UTF-8
+/// @param[in] userdata User data. Set to `nullptr` if you don't need it.
+///
+std::string ExpandFilePath(const std::string &filepath, void *userdata);
 
 bool ReadWholeFile(std::vector<unsigned char> *out, std::string *err,
                    const std::string &filepath, void *);