I've made a few more changes, upon further thought:

1. Our cleaner, modern way of wording rounding (e.g. https://eel.is/c++draft/expr.shift#3) is to say "rounded towards X".
   Saying "truncation towards zero" is redundant, and we don't need clarifying footnotes if we just word it properly.
2. The following explanation of a / b = (a/b)*b + a%b is totally tutorial-like. That's simply a consequence of % being the remainder of the division with said rounding mode, as the paragraph states. It also adds no new information to say that the behavior is undefined if the result cannot be represented in the result type; that's a general rule for [expr]. I think it's best to just keep this wording in a note, even if it's giving the user a math tutorial and restating something broadly known. \ref{expr} may be helpful though, lemme add that.