The Notes tab in a deal's right-hand sidebar is one shared, freeform note for the whole deal — formatting, lists, and all.
There's no Save button: your changes save automatically about a second and a half after you stop typing. A status indicator in the bottom-right of the panel shows Saving..., then Saved — or Save failed if something went wrong, in which case try again.
Because it's shared, anyone on the deal who can see it can also edit it — there's no per-person note.