Branch sheriff docs: add note about git fetch.

I couldn't check out the branch during my branch sheriff shift yesterday
until I figured out that I needed to run git fetch. This CL basically
copies the similar note from:
https://www.chromium.org/developers/how-tos/get-the-code/working-with-release-branches

Change-Id: I00a09a995ccedf49ea535f6ab3ed56d8c32eed13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2124306
Reviewed-by: John Budorick <[email protected]>
Commit-Queue: Justin Donnelly <[email protected]>
Cr-Commit-Position: refs/heads/master@{#754131}
diff --git a/docs/branch_sheriff.md b/docs/branch_sheriff.md
index 8b42d87..ec8b4ff 100644
--- a/docs/branch_sheriff.md
+++ b/docs/branch_sheriff.md
@@ -39,6 +39,12 @@
 
 > This only needs to be done once, though running it more than once won't hurt.
 
+You may also need to run:
+
+```
+  src $ git fetch
+```
+
 Once you've done that, you'll be able to check out branches:
 
 ```