Importing GitHub cross-repository PRs is not supported atm

[ci skip]

doc/workflow/importing/import_projects_from_github.md

@@ -11,8 +11,9 @@ At its current state, GitHub importer can import:
 - the pull requests (introduced in GitLab 8.4)
 - the wiki pages (introduced in GitLab 8.4)
 
-It is not yet possible to import your labels and milestones. We are working on
-improving this in the near future.
+It is not yet possible to import your labels, milestones and cross-repository
+pull requests (those from forks). We are working on improving this in the near
+future.
 
 The importer page is visible when you [create a new project][new-project].
 Click on the **GitHub** link and you will be redirected to GitHub for