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

gio/giotypes.h

@@ -526,7 +526,7 @@ typedef struct _GSubprocess                   GSubprocess;
  *
  * Options for launching a child process.
  *
- * Since: 2.36
+ * Since: 2.40
  */
 typedef struct _GSubprocessLauncher           GSubprocessLauncher;