core: Add documentation for ostree_commit_get_timestamp()
core: Add documentation for ostree_commit_get_timestamp()

Working on some rpm-ostree bits and was going to pass
this to the `chrono` crate and I forgot the format, went
to look at the docs.  Oops.

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree