The main purpose of the tag-info (and especially the tag-excerpt) is to help with tagging. This is especially important for the tags with ambiguous name. But usually tag-info also contains some brief definition of the area belonging under tag and it might be a good places where to collect a few links useful for somebody to get basic idea about the subject.1
However, MathOverflow has many tag-excerpts/tag-wikis where the text is simply copy-pasted from Wikipedia, nLab, MathWorld, EOM or some other similar site.2 I think that in such case source should be attributed. (I believe that in the case of Wikipedia it is even required by CC BY-SA.) Apart from that, if we consider tag-wiki as a good place to collect some basic information, adding links to further resources might also be useful for the readers.
Main question. If the text of the tag-wiki is taken from some website, should the tag-wiki include some kind of attribution? Also, what form should this attribution have? Is simply adding a link to the relevant article considered sufficient attribution?
1I find possibility of collecting links useful especially for tags on meta, where a tag often is related to some functionality of Stack Exchange site. For example, if a user is new to SEDE, I can simply direct them to the tag-info. And the comment/answer containing this link might be also useful for other users. If something changes or some new relevant information should be added, it is easier to edit tag-info rather then edit all answers which contained the relevant links.
2If you want to see some specific examples, you can look at my recent tag-wiki edits. (I have added a link to a few tag-wikis of this type.)