docs: Clarify %NULL argument to ostree_sysroot_new()
docs: Clarify %NULL argument to ostree_sysroot_new()

It’s equivalent to ostree_sysroot_new_default().

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1246
Approved by: cgwalters

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