Doc: small fixes

This commit adds the GTestSubprocessFlags enum to the docs, and fixes
several minor typos in various places.

https://bugzilla.gnome.org/show_bug.cgi?id=703254

gio/gasyncresult.c

@@ -62,7 +62,7 @@
  * |[
  * void _theoretical_frobnitz_async (Theoretical         *t,
  *                                   GCancellable        *c,
- *                                   GAsyncReadyCallback *cb,
+ *                                   GAsyncReadyCallback  cb,
  *                                   gpointer             u);
  *
  * gboolean _theoretical_frobnitz_finish (Theoretical   *t,

gio/gtask.c

@@ -46,7 +46,7 @@
  *     Eventually, you will call a method such as
  *     g_task_return_pointer() or g_task_return_error(), which will
  *     save the value you give it and then invoke the task's callback
- *     function (waiting until the next next iteration of the main
+ *     function (waiting until the next iteration of the main
  *     loop first, if necessary). The caller will pass the #GTask back
  *     to the operation's finish function (as a #GAsyncResult), and
  *     you can use g_task_propagate_pointer() or the like to extract