Bug 1312839 - Properly document pulse data ingestion config

The previous docs incorrectly said to use a settings_local.py file.
The correct way to define the Pulse URL is via an environment
variable.

docs/pulseload.rst

@@ -28,10 +28,10 @@ Vagrant, or in your ``config/settings_local.py`` file.  For example::
 To be able to ingest from exchanges, you need to create a Pulse user with
 `Pulse Guardian`_, so
 Treeherder can create your Queues for listening to the Pulse exchanges.  For
-this, you must specify the connection URL in your environment variables or
+this, you must specify the connection URL in the ``PULSE_DATA_INGESTION_CONFIG``
-``settings_local.py``.  For example::
+environment variable. e.g.::
 
-    PULSE_DATA_INGESTION_CONFIG = "amqp://mypulseuserid:mypassword@pulse.mozilla.org:5671/?ssl=1"
+    export PULSE_DATA_INGESTION_CONFIG="amqp://mypulseuserid:mypassword@pulse.mozilla.org:5671/?ssl=1"
 
 Ingesting Data
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