meson: rename target to update-dbus-docs

Very old versions of meson did not include the subdirectory name in the
target name, so we started adding various "top-level" custom targets in
subdirectories. This was nice because the main meson.build file wasn't
as cluttered. But then meson started including the subdir name in the
target name. So let's move the definition to the root so we can have all
targets named uniformly.