Add some autoassigner notes (#6154)

Author: jonmeow

docs/project/code_review.md

@@ -28,6 +28,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
     -   [What should be covered by a review?](#what-should-be-covered-by-a-review)
     -   [Writing review comments](#writing-review-comments)
     -   [Approving the change](#approving-the-change)
+    -   [Pausing reviews and reassigning PRs](#pausing-reviews-and-reassigning-prs)
 -   [Merging pull requests](#merging-pull-requests)
     -   [Merge commit descriptions](#merge-commit-descriptions)
 -   [Resolving an impasse or conflict](#resolving-an-impasse-or-conflict)
@@ -440,6 +441,18 @@ author can always come back to you if they have questions, and we can always
 revert changes if the resolution for some reason diverges wildly from your
 expectations.
 
+### Pausing reviews and reassigning PRs
+
+When temporarily unavailable to review, for example due to a vacation, reviewers
+can either mark themselves as
+[busy in GitHub](https://docs.github.com/en/account-and-profile/tutorials/personalize-your-profile#setting-a-status),
+or ask an admin to stop assignment (using
+["Never assign to certain team members"](https://docs.github.com/en/organizations/organizing-members-into-teams/managing-code-review-settings-for-your-team#configuring-auto-assignment)).
+
+A PR's reviewer can also be changed to a team, such as
+`carbon-language/toolchain-reviewers`, and it should be automatically
+reassigned. This can be done by anyone, not just a reviewer.
+
 ## Merging pull requests
 
 Pull requests are ready to be merged when reviewers have indicated they're happy