DOC: Try to clarify which dtype instaces are still allowed
DOC: Try to clarify which dtype instaces are still allowed

We still allow the "singleton" instances (which mainly applies to
our own dtypes), mainly because it wouldn't really do much good to
disallow them, even if they are not specific (since we don't enforce
the byte-order, but we never return non-native byte order for example).

numpy/numpy,numpy/numpy,numpy/numpy,numpy/numpy