[glib: 1/2] Add various missing nullable annotations

[Philip Withnall <pwithnall src gnome org>]

commit c686e1a04857e78e29c558657d32955ac8546a31
Author: Sebastian Dröge <sebastian centricular com>
Date:   Sat Oct 17 19:59:39 2020 +0300

    Add various missing nullable annotations

 glib/gkeyfile.c    |  2 +-
 glib/gmain.c       |  6 +++---
 gobject/gbinding.c | 14 ++++++++++++--
 3 files changed, 16 insertions(+), 6 deletions(-)
---
diff --git a/glib/gkeyfile.c b/glib/gkeyfile.c
index 7c81bf070..9d0215331 100644
--- a/glib/gkeyfile.c
+++ b/glib/gkeyfile.c
@@ -1693,7 +1693,7 @@ g_key_file_get_keys (GKeyFile     *key_file,
  *
  * Returns the name of the start group of the file. 
  *
- * Returns: The start group of the key file.
+ * Returns: (nullable): The start group of the key file.
  *
  * Since: 2.6
  **/
diff --git a/glib/gmain.c b/glib/gmain.c
index acbd0d4f0..4d057badf 100644
--- a/glib/gmain.c
+++ b/glib/gmain.c
@@ -884,7 +884,7 @@ g_main_context_pop_thread_default (GMainContext *context)
  * If you need to hold a reference on the context, use
  * g_main_context_ref_thread_default() instead.
  *
- * Returns: (transfer none): the thread-default #GMainContext, or
+ * Returns: (transfer none) (nullable): the thread-default #GMainContext, or
  * %NULL if the thread-default context is the global default context.
  *
  * Since: 2.22
@@ -2112,7 +2112,7 @@ g_source_set_name (GSource    *source,
  * Gets a name for the source, used in debugging and profiling.  The
  * name may be #NULL if it has never been set with g_source_set_name().
  *
- * Returns: the name of the source
+ * Returns: (nullable): the name of the source
  *
  * Since: 2.26
  **/
@@ -3098,7 +3098,7 @@ g_main_depth (void)
  *
  * Returns the currently firing source for this thread.
  * 
- * Returns: (transfer none): The currently firing source or %NULL.
+ * Returns: (transfer none) (nullable): The currently firing source or %NULL.
  *
  * Since: 2.12
  */
diff --git a/gobject/gbinding.c b/gobject/gbinding.c
index 015889b24..78a883075 100644
--- a/gobject/gbinding.c
+++ b/gobject/gbinding.c
@@ -754,7 +754,12 @@ g_binding_get_flags (GBinding *binding)
  *
  * Retrieves the #GObject instance used as the source of the binding.
  *
- * Returns: (transfer none): the source #GObject
+ * A #GBinding can outlive the source #GObject as the binding does not hold a
+ * strong reference to the source. If the source is destroyed before the
+ * binding then this function will return %NULL.
+ *
+ * Returns: (transfer none) (nullable): the source #GObject, or %NULL if the
+ *     source does not exist any more.
  *
  * Since: 2.26
  */
@@ -772,7 +777,12 @@ g_binding_get_source (GBinding *binding)
  *
  * Retrieves the #GObject instance used as the target of the binding.
  *
- * Returns: (transfer none): the target #GObject
+ * A #GBinding can outlive the target #GObject as the binding does not hold a
+ * strong reference to the target. If the target is destroyed before the
+ * binding then this function will return %NULL.
+ *
+ * Returns: (transfer none) (nullable): the target #GObject, or %NULL if the
+ *     target does not exist any more.
  *
  * Since: 2.26
  */