[perf] Update perfbot sheriff docs to include buildslave monitoring.

tools/perf/docs/perf_bot_sheriffing.md

 # Perf Bot Sheriffing
 
 The perf bot sheriff is responsible for keeping the bots on the chromium.perf
 waterfall up and running, and triaging performance test failures and flakes.
 
 **[Rotation calendar](https://calendar.google.com/calendar/embed?src=google.com_2fpmo740pd1unrui9d7cgpbg2k%40group.calendar.google.com)**
 
 ## Key Responsibilities
 
 *   [Handle Device and Bot Failures](#botfailures)
@@ skipping 36 matching lines @@
 You can see a list of all previously filed bugs using the
 **[Performance-BotHealth](https://bugs.chromium.org/p/chromium/issues/list?can=2&q=label%3APerformance-BotHealth)**
 label in crbug.
 
 Please also check the recent
 **[perf-sheriffs@chromium.org](https://groups.google.com/a/chromium.org/forum/#!forum/perf-sheriffs)**
 postings for important announcements about bot turndowns and other known issues.
 
 ##<a name="botfailures"></a> Handle Device and Bot Failures
 
+###<a name="offline"></a> Offline Buildslaves
+
+Some build configurations, in particular the perf builders and trybots, have
+multiple machines attached. If one or more of the machines go down, there are
+still other machines running, so the console or waterfall view will still show
+green, but those configs will run at reduced throughput. At least once during
+your shift, you should check the lists of buildslaves and ensure they're all
+running.
+
+*   [chromium.perf buildslaves](https://build.chromium.org/p/chromium.perf/buildslaves)
+*   [tryserver.chromium.perf buildslaves](https://build.chromium.org/p/tryserver.chromium.perf/buildslaves)
+
+The machines restart between test runs, so just looking for "Status: Not
+connected" is not enough to indicate a problem. For each disconnected machine,
+you can also check the "Last heard from" column to ensure that it's been gone
+for at least an hour. To get it running again,
+[file a bug](https://bugs.chromium.org/p/chromium/issues/entry?labels=Pri-1,Performance-BotHealth,Infra-Troopers,OS-?&comment=Hostname:&summary=Buildslave+offline+on+chromium.perf)
+against the current trooper.
+
 ###<a name="purplebots"></a> Purple bots
 
-When a bot goes purple, it's it's usually because of an infrastructure failure
+When a bot goes purple, it's usually because of an infrastructure failure
 outside of the tests. But you should first check the logs of a purple bot to
 try to better understand the problem. Sometimes a telemetry test failure can
 turn the bot purple, for example.
 
 If the bot goes purple and you believe it's an infrastructure issue, file a bug
 with
 [this template](https://bugs.chromium.org/p/chromium/issues/entry?labels=Pri-1,Performance-BotHealth,Infra-Troopers,OS-?&comment=Link+to+buildbot+status+page:&summary=Purple+Bot+on+chromium.perf),
 which will automatically add the bug to the trooper queue. Be sure to note
 which step is failing, and paste any relevant info from the logs into the bug.
 
@@ skipping 153 matching lines @@
 
 **[Pri-2 bugs](https://bugs.chromium.org/p/chromium/issues/list?can=2&q=label%3APerformance-BotHealth+label%3APri-2)**
 are for disabled tests. These should be pinged weekly, and work towards fixing
 should be ongoing when the sheriff is not working on a Pri-1 issue. Here is the
 [list of Pri-2 bugs that have not been pinged in a week](https://bugs.chromium.org/p/chromium/issues/list?can=2&q=label:Performance-BotHealth%20label:Pri-2%20modified-before:today-7&sort=modified).
 
 <!-- Unresolved issues:
 1. Do perf sheriffs watch the bisect waterfall?
 2. Do perf sheriffs watch the internal clank waterfall?
 -->