At a point of time it was possible to put wikicode into Mediawiki:Powersearch-ns to have links. English wikisource used to have a link that explained the namespaces.
Sometime in the past there has been a change that nullifies wikicode in that file, eg. [[Help:Namespaces|Namespaces]] rather than being a wikilink displays as
that raw text and bolded.
I do not see any other ready means to be able to put context sensitive help about namespaces in that section.
[I cannot find a category for mediawiki messages]