Improve bevy_winit documentation (#7609)

Redo of #7590 since I messed up my branch.

# Objective

- Revise docs.
- Refactor event loop code a little bit to make it easier to follow.

## Solution

- Do the above.

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### Migration Guide

- `UpdateMode::Reactive { max_wait: .. }` -> `UpdateMode::Reactive {
wait: .. }`
- `UpdateMode::ReactiveLowPower { max_wait: .. }` ->
`UpdateMode::ReactiveLowPower { wait: .. }`

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Co-authored-by: Sélène Amanita <134181069+Selene-Amanita@users.noreply.github.com>