Add new doc for how to snooze an alert

R=nednguyen@chromium.org

Bug: 842232
Change-Id: Idf15bbf2fe41f79b173b79ab5d290002d8ef65c7
Reviewed-on: https://chromium-review.googlesource.com/1057828Reviewed-by: Ned Nguyen <nednguyen@google.com>
Commit-Queue: Ned Nguyen <nednguyen@google.com>
Cr-Commit-Position: refs/heads/master@{#558350}

docs/speed/bot_health_sheriffing/how_to_snooze_an_alert.md

+# How to snooze an alert
+
+After addressing an alert, the next step is to snooze it. 

Snoozing an alert hides the alert, moving it to a collapsed section at the bottom of the "Consistent alerts" section until the specified time has expired. This acts as a signal to yourself and other sheriffs that no further action is necessarily until the alert becomes unsnoozed.
+
+To snooze an alert, click the "Snooze" button for the specified alert and then enter the number of minutes you'd like to snooze the alert for.
+
+![Snooze an alert](snooze_alert.png)
+
+![Alert snooze dialog](snooze_alert_dialog.png)
+
+Generally, you should snooze an alert for however long you think it will take for your fix to take effect and a successful run to complete, removing the alert from Sheriff-o-matic. If you're unsure of how long this should be, 24 hours (1440 minutes) is usually a safe default.

docs/speed/bot_health_sheriffing/what_test_is_failing.md

-# What story is failing?
+# How to determine what story is failing
 
 The first step in addressing a test failure is to identify what stories are failing.