Annotate the GString constructors

Otherwise the introspection scanner won't recognise them as
constructors, because the GString get_type function has a different
symbol prefix.

See: https://gitlab.gnome.org/GNOME/gobject-introspection/-/issues/399

glib/gstring.c

@@ -104,16 +104,15 @@ g_string_maybe_expand (GString *string,
 }
 
 /**
- * g_string_sized_new:
- * @dfl_size: the default size of the space allocated to
- *     hold the string
+ * g_string_sized_new: (constructor)
+ * @dfl_size: the default size of the space allocated to hold the string
  *
  * Creates a new #GString, with enough space for @dfl_size
  * bytes. This is useful if you are going to add a lot of
  * text to the string and don't want it to be reallocated
  * too often.
  *
- * Returns: the new #GString
+ * Returns: (transfer full): the new #GString
  */
 GString *
 g_string_sized_new (gsize dfl_size)
@@ -131,13 +130,13 @@ g_string_sized_new (gsize dfl_size)
 }
 
 /**
- * g_string_new:
+ * g_string_new: (constructor)
  * @init: (nullable): the initial text to copy into the string, or %NULL to
  *   start with an empty string
  *
  * Creates a new #GString, initialized with the given string.
  *
- * Returns: the new #GString
+ * Returns: (transfer full): the new #GString
  */
 GString *
 g_string_new (const gchar *init)
@@ -160,7 +159,7 @@ g_string_new (const gchar *init)
 }
 
 /**
- * g_string_new_len:
+ * g_string_new_len: (constructor)
  * @init: initial contents of the string
  * @len: length of @init to use
  *
@@ -172,7 +171,7 @@ g_string_new (const gchar *init)
  * responsibility to ensure that @init has at least @len addressable
  * bytes.
  *
- * Returns: a new #GString
+ * Returns: (transfer full): a new #GString
  */
 GString *
 g_string_new_len (const gchar *init,