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/gsignal.c

@@ -2112,7 +2112,8 @@ g_signal_chain_from_overridden (const GValue *instance_and_params,
 
 /**
  * g_signal_chain_from_overridden_handler: (skip)
- * @instance: the instance the signal is being emitted on.
+ * @instance: (type GObject.TypeInstance): the instance the signal is being
+ *    emitted on.
  * @...: parameters to be passed to the parent class closure, followed by a
  *  location for the return value. If the return type of the signal
  *  is #G_TYPE_NONE, the return value location can be omitted.
@@ -3141,7 +3142,8 @@ accumulate (GSignalInvocationHint *ihint,
 
 /**
  * g_signal_emit_valist: (skip)
- * @instance: the instance the signal is being emitted on.
+ * @instance: (type GObject.TypeInstance): the instance the signal is being
+ *    emitted on.
  * @signal_id: the signal id
  * @detail: the detail
  * @var_args: a list of parameters to be passed to the signal, followed by a