[tepl] FileMetadata: add API design note

[Sébastien Wilmet <swilmet src gnome org>]

commit b7ebf447889343d9ce5f39da5a1bca2d4f49e0e9
Author: Sébastien Wilmet <swilmet gnome org>
Date:   Wed Apr 8 18:50:40 2020 +0200

    FileMetadata: add API design note

 tepl/tepl-file-metadata.c | 8 ++++++++
 1 file changed, 8 insertions(+)
---
diff --git a/tepl/tepl-file-metadata.c b/tepl/tepl-file-metadata.c
index b5827d0..650a49a 100644
--- a/tepl/tepl-file-metadata.c
+++ b/tepl/tepl-file-metadata.c
@@ -41,6 +41,14 @@
  * metadata on disk, call tepl_file_metadata_save_async().
  */
 
+/* API design - additional notes:
+ *
+ * The values must be valid UTF-8 strings, not arbitrary byte string because
+ * G_FILE_ATTRIBUTE_TYPE_STRING is used. And also because it's convenient to
+ * have UTF-8 strings, in case they are displayed in the UI with GTK. Note that
+ * there is also G_FILE_ATTRIBUTE_TYPE_BYTE_STRING.
+ */
+
 /* TODO Better test how it works with remote files, with various protocols.
  * For example with an ftp://... location, there can be the error "The specified
  * location is not mounted". In that case we can either propagate the error or