Create crosslink between email pages

These two pages each need a sentence to help disambiguate their
purposes. They look really similar in search results, but I couldn't
find the one I needed to be reading, so I'm crosslinking!

doc/tools/email.md

@@ -11,6 +11,9 @@ GitLab provides a simple tool to administrators for emailing all users, or users
 a chosen group or project, right from the Admin Area. Users receive the email
 at their primary email address.
 
+For information about email notifications originating from GitLab, read
+[GitLab notification emails](../user/profile/notifications.md).
+
 ## Use-cases
 
 - Notify your users about a new project, a new feature, or a new product launch.

doc/user/profile/notifications.md

@@ -11,6 +11,9 @@ GitLab Notifications allow you to stay informed about what's happening in GitLab
 enabled, you can receive updates about activity in issues, merge requests, epics, and designs.
 Notifications are sent via email.
 
+For the tool that enables GitLab administrators to send messages to users, read
+[Email from GitLab](../../tools/email.md).
+
 ## Receiving notifications
 
 You receive notifications for one of the following reasons: