How should copied content be formatted when editing tag wikis?
Original is Best
I am sure many tag wikis are written in good faith, but there needs to be some guidance on how to do so.
In an ideal world, if you are not writing an original tag wiki, then do not suggest the edit. Within the pool of talent on SE there would be many capable people who can author original tag wiki content, it is a matter of encouraging people to do so and discouraging people to copy and paste content. Any original work on the site, as with posts, then falls under the licencing conditions of the site.
Whilst the preference is to have tag wikis to be original content, there are still many tag wikis being suggested that contain copied content. Frequently this content is copied and pasted with a link to the original site, but not formatted correctly.
When using content with attribution it's important to make it clear which part of the content is being copied and the way to define this is with the use of quotation marks or quotation mark up.
Plagiarised Tag Wiki Content
For example, this tag wiki wiki history:
The original wiki edit used content from another site without any link to the original site. The gives the appearance that the content is the original work of the editor. This is plagiarism.
Incorrectly formatted attributed content
For example, this tag wiki history:
The original wiki edit did provide a link to the original site, but none of the copied content is quoted. As it stands this gives the appearance that the editor is the author of the content and is providing a supplementary link to another site. (I am not suggesting this was the editor's intention!) This is plagiarism.
Correctly formatted content
There is no hard and fast rule of how to format copied content in tag wikis, the aim to clearly demonstrate whether the content is the original work of the wiki editor or from another author.
Using the previous example:
Any lengthy portions of copied content from the original site needs to be included within the quotation mark up with a link and description of the original content. In this case the original content is in the documentation at the github repo, in other cases the actual name of the site may be preferable.
With tag wikis that consist only of lengthy quotations, I prefer to add a sentence stating where the content is being sourced from, rather than including a link within the quotation as demonstrated in some examples further down.
So the resulting effort looks like this:
The following example has content that is paraphrased from a site and also quoted from that same site. The first two paragraphs are paraphrased, there is enough similarity between the wiki content and the original site, that attribution is required. By beginning the wiki with a referral to the original content, it is then inferred that all the following content is from this site, the quotation mark up demonstrating it is a copy and paste, with the link added at the beginning of the quotation for good measure to avoid any misunderstanding.
This example has original content followed by some quoted content. Here the copied content is clearly set apart from the original content using quotation mark up and it has a long link at the start of the quote to make it clear to readers where the quoted content is being sourced.
First and foremost, check if the content you're going to copy can be reused under CC-BY-SA 3.0 that SE uses at all!
Keep in mind that all texts are "all rights reserved" by default. And excerpts covered by fair use can only be used in a limited fashion - which is unacceptable in tag wikis because the entire text is freely edited1.
Even free licenses often have provisions that make them incompatible with CC-BY-SA. In general, you can import verbatim content that's under permissive licenses or a weaker or the same variant of CC.
If you're unsure, it's best not to guess around and just rewrite the text in your own words. This doesn't constitute copyright violation (yet).
If you still wish to give credit as a courtesy, it's better to state that it's been rewritten to avoid confusion.
CC-BY(-SA) and most other licenses require (or at least, request) attribution to the work being used. URL is commonly seen as adequate.
Attributing in the text itself is impractical in tag wikis (it'll clutter it and as the text is further edited, it'll become impossible to see what is from where). So, the optimal place to make an attribution is in the edit summary.
The edit history with the summaries is readily available at the bottom of the tag wiki page so the format satisfies attribution requirements.
In some rare cases, sources explicitly require attribution in a form other than URL (no licenses I know of except the obsolete 4-clause BSD do that by default, it must be stated explicitly). If such a requirement is impractical/impossible to satisfy with the summary, this means you shouldn't use such a source.
1Strictly speaking, it's unacceptable in Q&A, too, as it means that the entire content is unavailable under CC-BY-SA. But this will only be a concern for reusers and only if citations are built upon: while they are faithful and marked as citations, they are fair use.