While "Menu/Theme locations" was descriptive of what the choices listed were, in the context of editing an individual nav menu it is more descriptive to use a setting label that tells you what it is you are affecting for that nav menu, which is where it will be displayed. Technically multiple locations can be chosen, but in practice that appears to be rare enough that we can avoid an awkward "location(s)" type string.
props Fencer04 for the initial patch.
fixes#38525.
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