Bug 1863399 - Fix telemetry enabling documentation and move to source-docs. r=#thunderbird-reviewers

Telemetry is enabled by default in official builds since bug 1549886.

Differential Revision: https://phabricator.services.mozilla.com/D192875

--HG--
rename : mail/components/telemetry/README.md => docs/telemetry/index.md
extra : amend_source : 19d81317f961818e9682e0f8e7cda36df1c03508

docs/index.rst

@@ -16,6 +16,8 @@ Thunderbird Source Tree Documentation
 
    storybook/index
 
+   telemetry/index
+
 
 
 Indices and tables

docs/telemetry/index.md

@@ -37,12 +37,11 @@ to filter out histograms with a `TELEMETRY_TEST_` prefix).
 
 ## Compile-time switches
 
-Telemetry is not compiled in by default. You need to add the following line
-to your mozconfig:
+Telemetry is compiled in by default for Nightly and Official builds. To enable for
+unofficial builds, add the following line to your mozconfig (lack of a value is
+intentional):
 
-    export MOZ_TELEMETRY_REPORTING=1
-
-The nightly and release configs have this setting already (`$ grep -r MOZ_TELEMETRY_ mail/config/mozconfigs`).
+    ac_add_options MOZ_TELEMETRY_REPORTING=
 
 ## Runtime prefs for testing