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

glib/gmessages.c

@@ -942,7 +942,8 @@ static GSList *expected_messages = NULL;
 
 /**
  * g_logv:
- * @log_domain: the log domain
+ * @log_domain: (nullable): the log domain, or %NULL for the default ""
+ * application domain
  * @log_level: the log level
  * @format: the message format. See the printf() documentation
  * @args: the parameters to insert into the format string
@@ -1089,7 +1090,8 @@ g_logv (const gchar   *log_domain,
 
 /**
  * g_log:
- * @log_domain: the log domain, usually #G_LOG_DOMAIN
+ * @log_domain: (nullable): the log domain, usually #G_LOG_DOMAIN, or %NULL
+ * for the default
  * @log_level: the log level, either from #GLogLevelFlags
  *     or a user-defined level
  * @format: the message format. See the printf() documentation
@@ -1117,6 +1119,12 @@ g_log (const gchar   *log_domain,
   va_end (args);
 }
 
+/**
+ * g_return_if_fail_warning: (skip)
+ * @log_domain: (nullable):
+ * @pretty_function:
+ * @expression: (nullable):
+ */
 void
 g_return_if_fail_warning (const char *log_domain,
 			  const char *pretty_function,
@@ -1129,6 +1137,14 @@ g_return_if_fail_warning (const char *log_domain,
 	 expression);
 }
 
+/**
+ * g_warn_message: (skip)
+ * @domain: (nullable):
+ * @file:
+ * @line:
+ * @func:
+ * @warnexpr: (nullable):
+ */
 void
 g_warn_message (const char     *domain,
                 const char     *file,
@@ -1379,10 +1395,11 @@ escape_string (GString *string)
 
 /**
  * g_log_default_handler:
- * @log_domain: the log domain of the message
+ * @log_domain: (nullable): the log domain of the message, or %NULL for the
+ * default "" application domain
  * @log_level: the level of the message
- * @message: the message
- * @unused_data: data passed from g_log() which is unused
+ * @message: (nullable): the message
+ * @unused_data: (nullable): data passed from g_log() which is unused
  *
  * The default log handler set up by GLib; g_log_set_default_handler()
  * allows to install an alternate default log handler.