Mention rollup- early

Author: aidanhs

performing-a-rollup.md

@@ -12,8 +12,9 @@ be pushed to master. As PR volume increases this can scale poorly, especially gi
 the 2hr30min current CI duration for Rust.
 
 Enter rollups - low risk changes (often doc fixes or other non-functional changes)
-are marked with the `rollup` command to bors (e.g. `@bors r+ rollup` to approve a
-PR and mark as a rollup or `@bors rollup` to mark a previously approved PR).
+are marked with the `rollup` command to bors (`@bors r+ rollup` to approve a PR
+and mark as a rollup, `@bors rollup` to mark a previously approved PR, `@bors rollup-`
+to unmark as a rollup).
 'Performing a Rollup' then means collecting these changes into one PR and merging
 them all at once.