Docs: setting valid_type automatically makes the namespace dynamic

[superstar54]

From the comment by @sphuber in https://aiida.discourse.group/t/how-to-use-a-non-dynamic-input-namespace/511/4

Setting valid_type automatically makes the namespace dynamic, but this is not mentioned in the docs, thus confusing the developer as to why a non-dynamic namespace accepts an unknown number of inputs.

It would be great to add this.