Moves celery.utils.mail from internal reference to public reference

docs/internals/reference/index.rst

@@ -48,7 +48,6 @@
     celery.utils.timeutils
     celery.utils.encoding
     celery.utils.compat
-    celery.utils.mail
     celery.utils.patch
     celery.utils.serialization
     celery.utils.timer2

docs/reference/index.rst

@@ -21,6 +21,7 @@
     celery.task.http
     celery.schedules
     celery.signals
+    celery.utils.mail
     celery.exceptions
     celery.loaders
     celery.loaders.app