docs: explain mode-line state and update flow

Four variables hold the mode-line's runtime state: the cache, the rendered string, the stale-render flag, and the refresh timer. They get updated in a specific order across three different functions. Anyone reading `wttrin--mode-line-tooltip` for the first time hits a clever bit on every hover: the tooltip re-evaluates staleness and triggers a re-render of the icon if it flipped. Without a comment that ties this to the rest of the update flow, you have to trace through three other functions to understand why.

The new comment block above the defvars lays out the order and the hover-driven re-render in one place, so future readers don't have to reconstruct it from the call graph.

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