gotta agree with the others, implementing a specific function for chapter descriptions isn't worth the effort.
Actually, couldn't this easily be implemented by an author themselves by using the spoiler function? Like, just start off with a spoiler that you label 'description of chapter' or something and then readers could open that?
This is easy to do and can be easily replicated each chapter, or even be used as an announcement or authors note.
Not to mention not all webnovels have a standard length, I go around 2.5-3.5k words but someone might have a 1k word average, others may just be all overt he place, which is why we have such a wide birth when it comes to customizing things on our own terms with the SH editor and if you want something fancy, some unicode.
Some chapters do not need a description that can easily be remembered through just reading the chapter. Only in cases like people who post 10k word chapters would I ever think this idea would be necessary, and then again, I have yet to see a story with that high of a word average per chapter.