This workflow is somewhat confusing and could definitely benefit from an overhaul. It's easy to forget to question how informative the UI is when you already know how to use the page. So, let's start with understanding the entry points and UI of the page itself:
Visiting the tag info page for a tag with no content looks like this:
You can see this differs from your screenshot, which shows only the orange lower box while your suggested edit is held in the upper, white box.
Here is the header of a tag page:
Here's what a tag card shows when you hover over a tag when the tag has no excerpt:
I'm not sure how you got to the tag wiki editor.
- You could have started at the tag info page similar to the first image above which, somewhat unintuitively, shows two distinct boxes with missing info and how to create it and then clicked the button to "Create tag info" at the bottom of the page.
- You could have gone directly from the tag page for the tag using the "help us create it" link.
- You could have gone by clicking the "Help us edit this wiki" text on the tag card when you hovered over it somewhere on the site.
If the first, while not completely obvious, you at least get a hint there's two separate spots for text and the purpose of that text. If one of the other two, there's a lot less information to help you out.
Now, let's look at the tag wiki editor page:
While I'm totally willing to accept that the yellow box on the right wasn't visible to you - it's tucked away below everything on mobile or other narrow screens and it's kinda easy to miss if you're just expecting it to be the usual right sidebar - but I think there is plenty of guidance available, it's just hard to find.
The rest of the page is... inscrutable and needs a lot of work. Why does the "Usage guidance (excerpt)" box have a heading but the full tag wiki editor doesn't? If I didn't already know how to use this page, I would be really confused by this presentation.
Bare minimum, put a buffer between the two editor boxes and add a header to the lower one labeling it "Tag Wiki".
Once that's changed, start considering whether some of that info could be closer to the text entry fields. The short sentence about the usage guidance could easily live between the heading and the text box without changing the page height much. The longer info about the wiki could be split up so that a sentence briefly covers the purpose of the wiki in plain text. The yellow box can be repurposed to say "How to write a good Tag Wiki?" and focus on that information alone.