As a user, I don't remember all the namespace-numbers.
I suggest adding a link into the UI of the tool, that points to https://www.mediawiki.org/wiki/Special:MyLanguage/Manual:Namespace#Built-in_namespaces
It could be placed next to the message Comma-separated list of namespace IDs. Leave blank for all namespaces.
It could be labeled simply "List of default namespaces
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I've added a little help icon that goes to the on-wiki documentation. Hopefully that will suffice. I just thought that line was already pretty long, so adding a text-based link might further hurt the simplistic design.
Thanks for the fine suggestion!