[Update] Approve "Priority Sources for Tags"

[AdultSun]

Needs official approval:
https://guidelines.stashdb.org/docs/tags/assigning-tags/#priority-sources-for-tags

I came up with this priority ranking on my own for the Google Doc guidelines way back when, but it's always seemed pretty straightforward to me. Essentially, it places the full video as the primary source that trumps anything else that may be used for adding/changing tags. It also requests that edits explain what source they are using when editing a scene's tags so other voters/contributors will know whose source should take precedence in case there's some conflict.