Document preset application behaviour when using Add All

[drmandarin]

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Specifically with reference to the usability issue raised in HandBrake 3338, I'd like to request that the documentation is updated to detail how presets are applied when using Add All - most especially if, as currently, the application of preset (be it saved or modified or mixed) is counterintuitive.

Noting that the outcome of 3338 seems to be a pending change in the GUI, if this change is imminent and the solution results in a completely intuitive application of presets, this documentation enhancement may not be needed.