Docs: add note about translations screenshots to UI code review guide

Bug: none
Change-Id: I8cc1ad3b1731f824ff62472bfb2eb813636c8827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3122945
Reviewed-by: Peter Boström <pbos@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915701}

docs/ui/learn/bestpractices/prepare_for_code_review.md

@@ -28,4 +28,7 @@
   manually to double or triple their length.
 * When you post UI code for review, include screenshots. The best way to do this
   is posting before (if relevant) and after screenshots as attachments on the
-  linked bug. Mention this in the review.
+  linked bug. Mention this in the review.
+  * When adding or changing strings, presubmit will prompt you to upload
+    screenshots to facilitate translations. The associated hash can be used by
+    reviewers to [view the screenshots](https://docs.google.com/document/d/1nwYWDny20icMSpLUuV_LgrlbWKrYpbXOERUIZNH636o/edit#heading=h.ndzwtb9u4qzw).