Update docs to mention switching between mod_statusbar and mod_dock

[fantasai]

Figured out my statusbar sizing issue once I realized the date was missing: Notion was loading trayion as a dock instead of in the statusbar.

Which brings me to this issue: Section 3.6 of the docs should probably mention how to switch between mod_dock and mod_statusbar. :) (It wasn't hard to figure out using grep, tbh, but thought it might deserve a mention in the docs where we talk about the two.)