Add description for `mode` option. Fixes #6135
Add description for `mode` option. Fixes #6135

Changed from being a copy of `amd` description.

NekR/webpack,EliteScientist/webpack,NekR/webpack,SimenB/webpack,SimenB/webpack,EliteScientist/webpack,SimenB/webpack,webpack/webpack,SimenB/webpack,webpack/webpack,webpack/webpack,webpack/webpack