[MLv2] First pass of docs improvements in `metabase.lib.js` (#40679)

Goal is to properly document the interface between MLv2 and the FE. In
particular I want to call out the health and fate of each function. Some
of them only exist to support legacy compatibility, for example, and
that should be documented to discourage new usage and make clear the
conditions for removing the functions.

src/metabase/lib/filter/update.cljc

@@ -181,6 +181,7 @@
              query         (if (= unit breakout-unit)
                              query
                              (temporal-filter-update-breakouts query stage-number temporal-column breakout-unit))]
+         ;; TODO: This "same unit or multiple units" logic exists in `shared.ut` somewhere; reuse it here.
          (if (= (str start) (str end))
            ;; is the start and end are the same (in whatever the original unit was) then just do an "="
            (lib.filter/filter query stage-number (lib.filter/= temporal-column start))