Bug 1187879 - Clarify Telemetry pref default behavior in documentation. r=dexter

toolkit/components/telemetry/docs/preferences.rst

@@ -3,9 +3,15 @@ Preferences
 
 Telemetry behaviour is controlled through the preferences listed here.
 
-*Note:* On official builds (which define ``MOZILLA_OFFICIAL``), Telemetry is only initialized when ``MOZ_TELEMETRY_REPORTING`` is defined.
+Default behaviors
+-----------------
+
+On official builds (which define ``MOZILLA_OFFICIAL``), Telemetry is only initialized when ``MOZ_TELEMETRY_REPORTING`` is defined.
 Sending only happens on official builds with ``MOZ_TELEMETRY_REPORTING`` defined.
 
+Preferences
+-----------
+
 ``toolkit.telemetry.unified``
 
   This controls whether unified behavior is enabled. If true:
@@ -24,6 +30,10 @@ Sending only happens on official builds with ``MOZ_TELEMETRY_REPORTING`` defined
   If ``unified`` is on, this controls whether to record *extended* data.
   This preference is controlled through the `Preferences` dialog.
 
+  Note that the default value here of this pref depends on the define ``RELEASE_BUILD`` and the channel.
+  If ``RELEASE_BUILD`` is set, ``MOZ_TELEMETRY_ON_BY_DEFAULT`` gets set, which means this pref will default to ``true``.
+  This is overridden by the preferences code on the "beta" channel, the pref also defaults to ``true`` there.
+
 ``datareporting.healthreport.uploadEnabled``
 
   Send the data we record if user has consented to FHR. This preference is controlled through the `Preferences` dialog.