GFile: add GBytes version of _replace_contents_async()

https://bugzilla.gnome.org/show_bug.cgi?id=690525
Document clearly async functions not copying its args

Usually async methods copy/ref its arguments so caller can
forget about them. g_file_replace_contents_async() and
g_output_stream_write_async() are exceptions.

https://bugzilla.gnome.org/show_bug.cgi?id=690525