g_file_info_get/set_attribute_string*(): Document the UTF-8ness.

* gio/gfileattribute.c: (_g_file_attribute_value_get_string,
_g_file_attribute_value_set_string): These use
G_FILE_ATTRIBUTE_TYPE_STRING, which is documented as UTF-8, so
document these private functions as using UTF-8.
* gio/gfileinfo.c: (g_file_info_get_attribute_string,
g_file_info_set_attribute_string, and stringv versions):
Document that the strings are UTF-8 because the implementation uses
those private functions, that use UTF-8.

This helps language bindings (such as glibmm) whose API
distinguishes between known and unknown encodings.