I also used SharePoint 2010's wiki feature briefly. It was convenient to make it available since we were already using SharePoint for other things.
Is there a clean way to migrate 8126 pages and 229 files from SharePoint Foundation 2010's wiki to DokuWiki?
If you can retrieve all of your SharePoint wiki pages in their raw textual form, then I would simply copy them all into corresponding pages within DokuWiki based on their resource name. Pages would not format correctly and links would be broken, but you could repair this as you go over time. Search would still be able to help you locate the content you need.
I doubt that SharePoint wiki pages are stored as text, however. Even though you use a text editor window to create and update them, they are probably stored in some other format behind the scenes. Turn your SharePoint wiki into a static site, and then use Dokuwiki's interwiki feature to make it easy to link backwards to existing content. This preserves access to your existing content while allowing you to use Dokuwiki moving forward. Since you converted everything to a static site, you can take your SharePoint 2010 environment offline if you want to.
Are there any limitations (the number of files/entries/pages as an example) I should be aware of?
I'm not aware of any. The beauty of Dokuwiki is its simplicity.