[glib] Document exactly what g_time_val_to_iso8601() produces

[Matthias Clasen <matthiasc src gnome org>]

commit a0c755710cdf3e06622b34e10088d4209189e6f9
Author: Simon McVittie <simon mcvittie collabora co uk>
Date:   Tue Oct 18 11:14:20 2011 +0100

    Document exactly what g_time_val_to_iso8601() produces
    
    Bug: https://bugzilla.gnome.org/show_bug.cgi?id=537637
    Signed-off-by: Simon McVittie <simon mcvittie collabora co uk>

 glib/gtimer.c |   22 ++++++++++++++++++++--
 1 files changed, 20 insertions(+), 2 deletions(-)
---
diff --git a/glib/gtimer.c b/glib/gtimer.c
index f9f8570..6e962b4 100644
--- a/glib/gtimer.c
+++ b/glib/gtimer.c
@@ -475,8 +475,26 @@ g_time_val_from_iso8601 (const gchar *iso_date,
  * g_time_val_to_iso8601:
  * @time_: a #GTimeVal
  * 
- * Converts @time_ into an ISO 8601 encoded string, relative to the
- * Coordinated Universal Time (UTC).
+ * Converts @time_ into an RFC 3339 encoded string, relative to the
+ * Coordinated Universal Time (UTC). This is one of the many formats
+ * allowed by ISO 8601.
+ *
+ * ISO 8601 allows a large number of date/time formats, with or without
+ * punctuation and optional elements. The format returned by this function
+ * is a complete date and time, with optional punctuation included, the
+ * UTC time zone represented as "Z", and the @tv_usec part included if
+ * and only if it is nonzero, i.e. either
+ * "YYYY-MM-DDTHH:MM:SSZ" or "YYYY-MM-DDTHH:MM:SS.fffffZ".
+ *
+ * This corresponds to the Internet date/time format defined by
+ * <ulink url="https://www.ietf.org/rfc/rfc3339.txt";>RFC 3339</ulink>, and
+ * to either of the two most-precise formats defined by
+ * <ulink url="http://www.w3.org/TR/NOTE-datetime-19980827";>the W3C Note
+ * "Date and Time Formats"</ulink>. Both of these documents are profiles of
+ * ISO 8601.
+ *
+ * Use g_date_time_format() or g_strdup_printf() if a different
+ * variation of ISO 8601 format is required.
  *
  * Return value: a newly allocated string containing an ISO 8601 date
  *