[glib] Document how to unset attributes

[Holger Berndt <hb src gnome org>]

commit d1e5161ab0206adc6f17325152e337d44ae6ec73
Author: Holger Berndt <hb gnome org>
Date:   Thu Jun 3 03:22:36 2010 +0200

    Document how to unset attributes
    
    Unsetting won't work on all attributes. Currently, only
    metadata attributes are supported.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=620423

 gio/gfile.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gio/gfile.c b/gio/gfile.c
index a5d00fe..c67dab9 100644
--- a/gio/gfile.c
+++ b/gio/gfile.c
@@ -3828,6 +3828,9 @@ g_file_query_writable_namespaces (GFile         *file,
  * @error: a #GError, or %NULL
  * 
  * Sets an attribute in the file with attribute name @attribute to @value.
+ *
+ * Some attributes can be unset by setting @attribute to
+ * %G_FILE_ATTRIBUTE_TYPE_INVALID and @value to %NULL.
  * 
  * If @cancellable is not %NULL, then the operation can be cancelled by
  * triggering the cancellable object from another thread. If the operation