doc: various improvements

- GSubprocessLauncher exists since 2.40, not 2.36
- more logical order for g_markup functions
- fix short description of GMarkup
- GMarkupParser: specify that some parameters are NULL-terminated.
- g_string_new (NULL); is possible.
- other trivial fixes.

https://bugzilla.gnome.org/show_bug.cgi?id=728983

glib/gstring.c

@@ -132,7 +132,8 @@ g_string_sized_new (gsize dfl_size)
 
 /**
  * g_string_new:
- * @init: the initial text to copy into the string
+ * @init: (allow-none): the initial text to copy into the string, or %NULL to
+ * start with an empty string.
  *
  * Creates a new #GString, initialized with the given string.
  *