docs(reference): document the sub-second duration units and fractional values The Duration reference listed only s/m/h/d/w and described durations as integers. Both parsers the runtime uses accept more: duration-parse (used by spicepod runtime fields) and fundu 2.0.1 (used by acceleration duration params) both accept ns, microseconds, and ms, accept fractional and exponent forms, treat a bare number as seconds, and reject month/year. fundu is pinned at 2.0.1 in every release tag from v1.5.2 through v2.1.1 and on trunk, so this correction applies to all versioned snapshots.