Document that the get_default() functions for the various GIO modules will never return NULL

gio/gmemorymonitor.c

@@ -118,7 +118,7 @@ static guint signals[LAST_SIGNAL] = { 0 };
  *
  * Gets a reference to the default #GMemoryMonitor for the system.
  *
- * Returns: (transfer full): a new reference to the default #GMemoryMonitor
+ * Returns: (not nullable) (transfer full): a new reference to the default #GMemoryMonitor
  *
  * Since: 2.64
  */

gio/gnetworkmonitor.c

@@ -83,7 +83,8 @@ static guint signals[LAST_SIGNAL] = { 0 };
  *
  * Gets the default #GNetworkMonitor for the system.
  *
- * Returns: (transfer none): a #GNetworkMonitor
+ * Returns: (not nullable) (transfer none): a #GNetworkMonitor, which will be
+ *     a dummy object if no network monitor is available
  *
  * Since: 2.32
  */

gio/gproxyresolver.c

@@ -72,7 +72,8 @@ g_proxy_resolver_default_init (GProxyResolverInterface *iface)
  *
  * Gets the default #GProxyResolver for the system.
  *
- * Returns: (transfer none): the default #GProxyResolver.
+ * Returns: (not nullable) (transfer none): the default #GProxyResolver, which
+ *     will be a dummy object if no proxy resolver is available
  *
  * Since: 2.26
  */

gio/gsettingsbackend.c

@@ -1001,7 +1001,9 @@ g_settings_backend_verify (gpointer impl)
  *
  * The user gets a reference to the backend.
  *
- * Returns: (transfer full): the default #GSettingsBackend
+ * Returns: (not nullable) (transfer full): the default #GSettingsBackend,
+ *     which will be a dummy (memory) settings backend if no other settings
+ *     backend is available.
  *
  * Since: 2.28
  */

gio/gtlsbackend.c

@@ -98,7 +98,8 @@ g_tls_backend_default_init (GTlsBackendInterface *iface)
  *
  * Gets the default #GTlsBackend for the system.
  *
- * Returns: (transfer none): a #GTlsBackend
+ * Returns: (not nullable) (transfer none): a #GTlsBackend, which will be a
+ *     dummy object if no TLS backend is available
  *
  * Since: 2.28
  */

gio/gvfs.c

@@ -342,7 +342,8 @@ g_vfs_parse_name (GVfs       *vfs,
  *
  * Gets the default #GVfs for the system.
  *
- * Returns: (transfer none): a #GVfs.
+ * Returns: (not nullable) (transfer none): a #GVfs, which will be the local
+ *     file system #GVfs if no other implementation is available.
  */
 GVfs *
 g_vfs_get_default (void)