Use the name of the linked containers instead of localhost

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1917#note_2833978

doc/ci/services/mysql.md

@@ -24,12 +24,15 @@ variables:
 And then configure your application to use the database, for example:
 
 ```yaml
-Host: localhost
+Host: mysql
 User: root
 Password: mysql_strong_password
 Database: el_duderino
 ```
 
+If you are wondering why we used `mysql` for the `Host`, read more at
+[How is service linked to the build](../docker/using_docker_images.md#how-is-service-linked-to-the-build).
+
 You can also use any other docker image available on [Docker Hub][hub-mysql].
 For example, to use MySQL 5.5 the service becomes `mysql:5.5`.
 

doc/ci/services/postgres.md

@@ -24,12 +24,15 @@ variables:
 And then configure your application to use the database, for example:
 
 ```yaml
-Host: localhost
+Host: postgres
 User: runner
 Password:
 Database: nice_marmot
 ```
 
+If you are wondering why we used `postgres` for the `Host`, read more at
+[How is service linked to the build](../docker/using_docker_images.md#how-is-service-linked-to-the-build).
+
 You can also use any other docker image available on [Docker Hub][hub-pg].
 For example, to use PostgreSQL 9.3 the service becomes `postgres:9.3`.