Since Emacs 25, single and double quote in the *HELP* and *MESSAGES* buffers are transliterated to their curved equivalents. This is usually, but not always, what you want. It turns out you can control whether or not the transliteration takes place but it’s not obvious how to do it.
Over at A Scripter’s Notes there’s a post that describes how to control the transliteration of quotes. It’s not too hard but it is a little fiddly. You won’t have to worry about this unless you’re writing functions or apps that other people are going to be using and then, truth to tell, only if you have a perfectionist approach to the presentation of your text output.
Sadly, I do have that approach so I’m glad to know how to deal with the issue. If you’re like me, take a look at the post.