Splatalogue: document export_limit

Not clear why the return line limit is a config item rather than a kwarg with a default, but in either case it should better be documented as it wasn't in fact easy to find the docstring with the kwarg name `export_limit`

cc @keflavich

Search the repository for `export_limit` and the Splatalogue client’s docstring to find where this configuration option is documented. Read the surrounding API documentation first, then make the option and its return-line limit easy to find; done means users can discover `export_limit` without relying on an obscure docstring.

Written by the indexing model from the issue text.