glib: Add missing (nullable) and (optional) annotations

Add various (nullable) and (optional) annotations which were missing
from a variety of functions. Also port a couple of existing (allow-none)
annotations in the same files to use (nullable) and (optional) as
appropriate instead.

Secondly, add various (not nullable) annotations as needed by the new
default in gobject-introspection of marking gpointers as (nullable). See
https://bugzilla.gnome.org/show_bug.cgi?id=729660.

This includes adding some stub documentation comments for the
assertion macro error functions, which weren’t previously documented.
The new comments are purely to allow for annotations, and hence are
marked as (skip) to prevent the symbols appearing in the GIR file.

https://bugzilla.gnome.org/show_bug.cgi?id=719966

gobject/gclosure.c

@@ -319,7 +319,8 @@ g_closure_set_meta_va_marshal (GClosure       *closure,
 /**
  * g_closure_set_meta_marshal: (skip)
  * @closure: a #GClosure
- * @marshal_data: context-dependent data to pass to @meta_marshal
+ * @marshal_data: (closure meta_marshal): context-dependent data to pass
+ *  to @meta_marshal
  * @meta_marshal: a #GClosureMarshal function
  *
  * Sets the meta marshaller of @closure.  A meta marshaller wraps
@@ -360,9 +361,11 @@ g_closure_set_meta_marshal (GClosure       *closure,
 /**
  * g_closure_add_marshal_guards: (skip)
  * @closure: a #GClosure
- * @pre_marshal_data: data to pass to @pre_marshal_notify
+ * @pre_marshal_data: (closure pre_marshal_notify): data to pass
+ *  to @pre_marshal_notify
  * @pre_marshal_notify: a function to call before the closure callback
- * @post_marshal_data: data to pass to @post_marshal_notify
+ * @post_marshal_data: (closure post_marshal_notify): data to pass
+ *  to @post_marshal_notify
  * @post_marshal_notify: a function to call after the closure callback
  *
  * Adds a pair of notifiers which get invoked before and after the
@@ -417,7 +420,7 @@ g_closure_add_marshal_guards (GClosure      *closure,
 /**
  * g_closure_add_finalize_notifier: (skip)
  * @closure: a #GClosure
- * @notify_data: data to pass to @notify_func
+ * @notify_data: (closure notify_func): data to pass to @notify_func
  * @notify_func: the callback function to register
  *
  * Registers a finalization notifier which will be called when the
@@ -453,7 +456,7 @@ g_closure_add_finalize_notifier (GClosure      *closure,
 /**
  * g_closure_add_invalidate_notifier: (skip)
  * @closure: a #GClosure
- * @notify_data: data to pass to @notify_func
+ * @notify_data: (closure notify_func): data to pass to @notify_func
  * @notify_func: the callback function to register
  *
  * Registers an invalidation notifier which will be called when the
@@ -921,7 +924,7 @@ _g_closure_set_va_marshal (GClosure       *closure,
 /**
  * g_cclosure_new: (skip)
  * @callback_func: the function to invoke
- * @user_data: user data to pass to @callback_func
+ * @user_data: (closure callback_func): user data to pass to @callback_func
  * @destroy_data: destroy notify to be called when @user_data is no longer used
  *
  * Creates a new closure which invokes @callback_func with @user_data as
@@ -949,7 +952,7 @@ g_cclosure_new (GCallback      callback_func,
 /**
  * g_cclosure_new_swap: (skip)
  * @callback_func: the function to invoke
- * @user_data: user data to pass to @callback_func
+ * @user_data: (closure callback_func): user data to pass to @callback_func
  * @destroy_data: destroy notify to be called when @user_data is no longer used
  *
  * Creates a new closure which invokes @callback_func with @user_data as
@@ -1496,7 +1499,8 @@ g_cclosure_marshal_generic (GClosure     *closure,
  * @return_value: (allow-none): a #GValue to store the return
  *  value. May be %NULL if the callback of @closure doesn't return a
  *  value.
- * @instance: the instance on which the closure is invoked.
+ * @instance: (type GObject.TypeInstance): the instance on which the closure is
+ *  invoked.
  * @args_list: va_list of arguments to be passed to the closure.
  * @marshal_data: (allow-none): additional data specified when
  *  registering the marshaller, see g_closure_set_marshal() and