Change docs to support paralel external and internal issue trackers

doc/integration/external-issue-tracker.md

@@ -4,14 +4,12 @@ GitLab has a great issue tracker but you can also use an external one such as
 Jira, Redmine, or Bugzilla. Issue trackers are configurable per GitLab project and allow
 you to do the following:
 
-- the **Issues** link on the GitLab project pages takes you to the appropriate
-  issue index of the external tracker
-- clicking **New issue** on the project dashboard creates a new issue on the
-  external tracker
 - you can reference these external issues inside GitLab interface
   (merge requests, commits, comments) and they will be automatically converted
   into links
 
+You can have enabled both external and internal GitLab issue trackers in parallel. The **Issues** link always opens the internal issue tracker and in case the internal issue tracker is disabled the link is not visible in the menu.
+
 ## Configuration
 
 The configuration is done via a project's **Services**.

doc/user/project/integrations/bugzilla.md

@@ -20,10 +20,12 @@ Once you have configured and enabled Bugzilla:
 ## Referencing issues in Bugzilla
 
 Issues in Bugzilla can be referenced in two alternative ways:
-1. `#<ID>` where `<ID>` is a number (example `#143`)
+1. `#<ID>` where `<ID>` is a number (example `#143`).
 2. `<PROJECT>-<ID>` where `<PROJECT>` starts with a capital letter which is
   then followed by capital letters, numbers or underscores, and `<ID>` is
   a number (example `API_32-143`).
 
+We suggest using the longer format if you have both internal and external issue trackers enabled. If you use the shorter format and an issue with the same ID exists in the internal issue tracker the internal issue will be linked.
+
 Please note that `<PROJECT>` part is ignored and links always point to the
 address specified in `issues_url`.

doc/user/project/integrations/redmine.md

@@ -30,5 +30,7 @@ Issues in Redmine can be referenced in two alternative ways:
   then followed by capital letters, numbers or underscores, and `<ID>` is
   a number (example `API_32-143`).
 
+We suggest using the longer format if you have both internal and external issue trackers enabled. If you use the shorter format and an issue with the same ID exists in the internal issue tracker the internal issue will be linked.
+
 Please note that `<PROJECT>` part is ignored and links always point to the
 address specified in `issues_url`.