[speed docs] Sheriff docs - auto description

It wasn't obvious to me that leaving the description blank
means it automatically gets the text "See the link to graphs
below." Updating docs.

Change-Id: I20b00383e05a7aebea27735c4e14e690817a3f77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1809113Reviewed-by: Annie Sullivan <sullivan@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697648}

docs/speed/perf_regression_sheriffing.md

@@ -63,6 +63,8 @@ below it the dashboard shows graphs of all the alerts checked in that table.
     * If one of the alerts already has a bug id, click "existing bug" and use
       that bug id.
     * Otherwise click "new bug".
+    * Only add a description if you have additional context. Otherwise a default
+      description will be automatically added when left blank.
 5. **Look at the revision range** for the regression. You can see it in the
    tooltip on the graph. If you see any likely culprits, cc the authors on the
    bug.