The litmus test on SE is effectively how people vote on your post, which in your case would tend to indicate that most readers don't mind much! (There's probably some amount of drive-by upvoting involved, as pointed out by muru. Upvoting and downvoting also tend to be unbalanced since the latter has a cost: it usually takes more incorrectness to get an answer downvoted, than it takes correctness and usefulness to get it upvoted. One of my most-upvoted answers was incorrect at first, and got many of its upvotes before I corrected it...)
As far as best practice goes, I would consider three aspects:
- effort
- sources
- investigation
Effort
Posting an answer to a question (and even posting a question) involves a variable amount of effort, depending on the question, the answer and the answerer. In my mind, any person who helps reduce the amount of effort involved in answering a question deserves some credit: thus if someone points something out to me in a comment on one of my answers, I'll generally credit that person in my update which takes that comment into account. In this instance, whoever wrote or updated the Wikipedia article which pointed you in the right direction probably deserves some credit by that measure. What's more, you can do this in a way which doesn't affect the readability of your answer much:
Wikipedia indicates that the "u" stands for "unlimited", based on the fact that the random_read_unlimited function name seems to indicate that the etymology of the letter u in urandom comes from unlimited.
Sources
I agree that pointing at definitive sources is better than pointing at intermediaries, so it's useful to have the link to the source of the function in question. It's even more useful to have the link to Ted Ts'o's message on sci.crypt which Stéphane Chazelas dug up.
Nevertheless I think the Wikipedia article is also a source, and not just an intermediary, since that's where the interpretation is given that 'u' stands for "unlimited".
Investigation
Finally, in many of these types of answers (historical, or digging into a complex technical issue), the investigative process is as interesting as the answer — in some cases even more interesting! So it's always useful to include that as far as I'm concerned, at least as long as it doesn't affect the readability of the answer. One useful approach can be the journalistic pyramid, or tl;dr: you give the answer in brief as the first paragraph, then add more and more detail in later paragraphs.
You'll find loads of examples of different styles on the site. Some users who tend to document their answers extensively are Stéphane Chazelas, Gilles, JdeBP and Thomas Dickey; it's no accident that they have some very highly-upvoted answers!
