Add detail around 'Build Key' within the GitLab steps.

doc/user/project/integrations/bamboo.md

@@ -41,8 +41,7 @@ service in GitLab.
 1. Click 'Atlassian Bamboo CI'
 1. Select the 'Active' checkbox.
 1. Enter the base URL of your Bamboo server. 'https://bamboo.example.com'
-1. Enter the build key from your Bamboo build plan. Build keys are a short,
-   all capital letter, identifier that is unique. It will be something like PR-BLD
+1. Enter the build key from your Bamboo build plan. Build keys are typically made up from the Project Key and Plan Key that are set on project/plan creation and seperated with a '-' for example  **PROJ-PLAN**. This is a short, all capital letter, identifier that is unique. When viewing a plan within Bamboo, the build key is also shown in the browser URL for example https://bamboo.example.com/browse/PROJ-PLAN
 1. If necessary, enter username and password for a Bamboo user that has
    access to trigger the build plan. Leave these fields blank if you do not require
    authentication.