Fix some documentation issue in glib/gstrfuncs.c

Apparently, the documentation of g_strcanon() was not really cristal
clear, so this new code sample try to make it clear the fact that we
are working on the given string and not a copy. Moreover, it provides
a way to keep the original string at once.

Fix #29

glib/gstrfuncs.c

@@ -1992,6 +1992,13 @@ g_strncasecmp (const gchar *s1,
  *   g_ascii_strup (g_strdelimit (str, "abc", '?'))
  * ]|
  *
+ * In order to modify a copy, you may use `g_strdup()`:
+ * |[<!-- language="C" -->
+ *   reformatted = g_strdelimit (g_strdup (const_str), "abc", '?');
+ *   ...
+ *   g_free (reformatted);
+ * ]|
+ *
  * Returns: @string
  */
 gchar *
@@ -2029,6 +2036,13 @@ g_strdelimit (gchar       *string,
  *   g_ascii_strup (g_strcanon (str, "abc", '?'))
  * ]|
  *
+ * In order to modify a copy, you may use `g_strdup()`:
+ * |[<!-- language="C" -->
+ *   reformatted = g_strcanon (g_strdup (const_str), "abc", '?');
+ *   ...
+ *   g_free (reformatted);
+ * ]|
+ *
  * Returns: @string
  */
 gchar *