docs: Expand introduction to mention using async calls over sync ones

docs: Expand introduction to mention using async calls over sync ones

As discussed on the mailing list (see the whole thread):
    https://mail.gnome.org/archives/desktop-devel-list/2015-February/msg00126.html

Expand the GIO documentation introduction to talk a little about when to
use async and sync functions, and how the former should almost always be
preferred over the latter.

Link to this from the GFile documentation, which is an entry point for a
lot of async calls.

https://bugzilla.gnome.org/show_bug.cgi?id=744722