[Spec] Document browserSignals.* for priority signals

[caraitto]

Several browser-generated prioritySignals are currently missing from the spec, and should be added:

• browserSignals.one
• browserSignals.basePriority
• browserSignals.firstDotProductPriority
• browserSignals.ageInMinutes
• browserSignals.ageInMinutesMax60
• browserSignals.ageInHoursMax24
• browserSignals.ageInDaysMax30
• browserSignals.ageInDays

[caraitto]

Here's the relevant section of the explainer: https://github.com/WICG/turtledove/blob/main/FLEDGE.md#35-filtering-and-prioritizing-interest-groups:~:text=The%20browser%2Dgenerated%20prioritySignals%20object%20contains%20the%20following%20values%3A

[caraitto]

@MattMenke2 @qingxinwu