Currently, the screen edge gesture shows a "glow" effect at the corresponding edge. This is hardly visible. Also the action is only triggered when the finger is moved fast enough and far enough - i.e. "move the finger from the edge to the center of the screen in less than 0.1 seconds". If you don't know how to trigger the gesture, it is hard to do so voluntarily. Possible improvement: As long as the touch is not released, expand the area, if it is large enough show a text stating the configured action (e.g. "Lock screen"). Allow the action to be stopped by "retracting".
The general problem with touch gestures is that you need to know them. No addition of text will fix this general problem. if we look at what others do, I think we already provide more visual hints than what is common. No system provides textual hints. Thus this won't be implemented.
If I look at my phone (Android), I get a very strong visual hint. Please read https://bugs.kde.org/show_bug.cgi?id=380428 to get an impression what the effect of the current scheme is.