expand the meaning of {beta,stable}-nominated

Author: tshepang

src/release/issue-triaging.md

@@ -135,9 +135,11 @@ There are many different labels that can be applied to issues.
     - `S-has-bisection`: A bisection has been performed and the bisection result is convincing.
     - `S-waiting-on-LLVM`: Waiting on upstream LLVM PR or fix.
     - `S-tracking-forever`: The tracking issue is never intended to be closed.
-- [`beta-nominated`]: Tracks changes which need to be [backported to beta].
-- [`stable-nominated`]: Tracks changes which need to be [backported to stable] (in anticipation of a point release).
-- [relnotes]: Changes that should be documented in the release notes of the next release.
+- [`beta-nominated`]: Tracks changes nominated for being [backported to beta]. If the relevant team(s) approve the backport, `T-release` will usually handle the backport.
+- [`stable-nominated`]: Tracks changes nominated for being [backported to stable], in anticipation of a point release. Similar to [`beta-nominated`], if the relevant team(s) approve the backport, `T-release` will usually handle the backport.
+- [relnotes]:
+  - It can be used to tag a PR w/ relnotes, which will make triagebot create a new relnotes issue.
+  - It also marks relnotes issues, so it can be processed by T-release relnotes tooling
 - [metabug]: Tracks other bugs.
 
 ### Relnotes issues