In the portage (5) manpage, the priority entry for repos.conf doesn't say which way it goes (higher number = preferred), and in general "lacks behavior specificity".
The (newer) binrepos.conf priority entry specifies higher priority preferred, and a good start would be simply copying it to the repos.conf priority entry, but I expect the below suggestions apply to both (I only use repos.conf, not the newer binrepos.conf, so far), as well.
Something about range, including specific mention that negative numbers work too (it hadn't occurred to me to try until I saw it somewhere) and suggesting them for repos that are always to be overridden (aka repos of last resort) by other repos if the package exists in more than one, would also be helpful.
Finally, some hint about what happens if two repos appear at the same priority would be useful. Is the preference effectively random in that case? Is the first or last in listed or alphabetical order preferred? Does repos.conf as a single file vs. subdir of files change that? Is same-repo preferred for a dep and does that override the above cases if they're the same priority? Etc.
FWIW, here, single repo/default section per file. gentoo is my repo of last resort, priority -9999. My local repo is highest priority, 9999, and other repos (kde, qt, whatever layman repos I'm experimenting with ATM) take the priority = 100 set in my default section/file. So there are generally two repos, sometimes more, at the middle 100 priority, and reading the docs I've always wondered what would happen if say both the qt and kde repos carried a new qt that was required by the live-git kde -9999 packages I run from the kde overlay but wasn't in the gentoo repo yet. (Real case at times, except that the kde repo doesn't carry the packages, requiring users to add the qt repo too when upstream requires a newer qt than is in gentoo. So I've wondered what if the kde repo carried it too and I had both repos at the same priority?)