UDF docs: clarify that type annotations are important

[soxofaan]

triggered from this forum thread: https://discuss.eodc.eu/t/udf-on-vector-cube/1142

it's not clear from the current UDF docs (especially the signatures section), that the type annotations are vital to make sure the UDF detection works

The signatures are rendered in Sphinx API style, but that's probably too abstract for a user to translate back to the expected python code