Make activation instructions more explicit

Author: c-w

app/authentification/templates/acc_active_email.html

@@ -1,5 +1,5 @@
 {% autoescape off %}
 Hi {{ user.username }},
-Please click on the link to confirm your email,
+Please click on the link to confirm your email and activate your Doccano account:
 {{ scheme }}://{{ domain }}{% url 'activate' uidb64=uid token=token %}
 {% endautoescape %}