Inserting a comment in the multi-line formula editor in Numbers

There is no inherent support for comments in formulas.

There are two workarounds, though.

The recommended/expected path for comments is via Insert -> Comment (Command-Shift-K), which applies a comment tag to the cell itself (not embedded within the formula).

Comments can be used for any purpose, including explanation of the cell's function, but it does have the side effect of leaving comment tags (the yellow triangle in the upper-right corner as you can see here:

The other option is a kind of hack in the formula itself using something like an IF() statement to wrap the comment so that it doesn't interfere with the calculation, e.g.:

In this example I've simply appended an IF() statement as an addition to the actual formula D4*B2*RAND().

Since the first parameter to IF() is TRUE(), it will always return 0 (the if-true parameter), thereby adding 0 to the result so it doesn't affect the outcome. The comment itself is embedded in the if-false parameter, which will never get executed, so it can say what you want.