This will not render correctly when using numpydoc. You should surround the colon with spaces and write only a short description of the type afterwards. On the next line, an indented and longer description begins and can span multiple (indented) lines. For example, you could write

    dtype : `str`
        A numpy dtype string (e.g. 'float32') indicating this spaces's dtype,
        or `None` for a dtype-agnostic space.