I wanted to add missing docs for space-evenly, so I simply opened css specs https://drafts.csswg.org/css-align-3/.
While I was on it, I tried to simplify space-around description to not have to compare each space-* to each others.
Finally, I adjusted flex-* and center by just ripping css specs like I did for the other desc.
Feel free to reject some parts of this update. I think some other pages needs the same update.
