Also updated the text for celery beat in the installation chapter

Author: mitsuhiko

docs/installation.rst

@@ -326,8 +326,10 @@ Sentry also needs a cron process which is called "celery beat":
 
   SENTRY_CONF=/etc/sentry sentry celery beat
 
-Make sure to only run one of them at the time or you will see unnecessary
-extra tasks being pushed onto the queues.
+It's recommended to only run one of them at the time or you will see
+unnecessary extra tasks being pushed onto the queues but the system will
+still behave as intended if multiple beat processes are run.  This can be
+used to achieve high availability.
 
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