People don’t read manuals and walls of texts, and even sometimes even a single paragraph of explanations.
What I do on one of my forums is sometimes publish a topic that consists of a simple reminder that a feature exists and invite people to use it. I tend to make it the shortest and clearest as possible.
Examples:
I think the most important is the topic’s title because most people won’t even read the topic.
So you could try something like that, with a topic’s title kind of like
Reminder: you can quote someone simply by selecting a part of their message and click on the “quote” pop-up.
And paste a screenshot of the feature in the post’s content.
(I’m not a native English speaker, so the title could certainly be improved).
Also, I sometimes add an emoji in the title (at the beginning or the end) to draw attention. In your case, maybe a 
or a 
?
Anyway, it might be worth a try…