document triagebot backport command

Author: apiraino

src/triagebot/zulip-commands.md

@@ -40,6 +40,7 @@ Note that the impersonation functionality is intended for inspecting the status
 - *Rust Project Goals* commands serve for controlling Rust Project Goal tracking.
   - `@triagebot ping-goals <threshold> <next-update>`: For use by the goals team to ping goal owners on Zulip to give an update on their goal. Will not ping if there has been a comment in `<threshold>` days. `<next-update>` is a string to say when the next blog update will start.
 - `@triagebot docs-update`: Generates a Pull Request ([example](https://github.com/rust-lang/rust/pull/141923)) to update the documentation submodules. See [Documentation Updates](doc-updates.md).
+- `@triagebot backport [stable | beta ] [approve | decline ] <PR>` (example: "@triagebot backport beta approve 123456") Will post a comment on GitHub to approve or decline a PR backport (see [Backports](./compiler/backports.md).
 
 ## Implementation