There's three major sections to the Effect menu: Illustrator Effects, Photoshop Effects, and Additional Effects.

Additional Effects is the domain of plug-in effects, and properly documenting these is of course in the hands of whoever wrote the plug-in.

Photoshop Effects is full of bitmap effects. There's a page in the docs that runs down the whole list and describes every one of them with a sentence or three. Great! https://helpx.adobe.com/illustrator/using/summary-effects.html

Illustrator Effects, though? Some of it's documented. There's a page about the Scribble effect, which is also for some reason about the Object Mosaic menu item. There's a page about the Drop Shadow, Inner/Outer Glow, and Feather effect. But where's the rest? There's an entirely unrelated section about the new 3d effects halfway across the manual, and nothing about the "classic" ones and the reasons you might still choose to use them now. How do the SVG effects work? What are the various Pathfinder effects useful for? What on earth is Outline Object supposed to do?

Seriously I have been using this thing for 25 years and it is only today that I learnt what Outline Object is for because someone posted a question about this in the Illustrator subreddit; someone else posted a link to a video Monika Gause made explaining this and lemme tell you, I never would have stumbled across what it's useful for in a million years of using Illustrator normally.

I assume there was documentation for this once, but it's vanished in manual rewrites. I know other things that are completely undocumented now like "holding down the ` key while transforming shapes switches to transforming their fill patterns, as long as you're not using transformation tools written after this fact vanished from the manual like the Free Transform tool". So much knowledge of how this program works is now lore passed down from one artist to another and that's really kind of annoying. Please dig up all the old docs you can find, figure out what's gone missing, figure out what menu items and tools have no docs, and write some new ones.