Update Github Importer documentation

Update Github Importer documentation to include new importers added in:
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48561
- https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48632

Related to:
- https://gitlab.com/gitlab-org/gitlab/-/issues/288826

doc/user/project/import/github.md

@@ -23,6 +23,8 @@ The following aspects of a project are imported:
 - Labels (GitLab.com & 8.7+)
 - Release note descriptions (GitLab.com & 8.12+)
 - Pull request review comments (GitLab.com & 10.2+)
+- Pull request reviews (GitLab.com & 13.7+)
+- Pull request "merged by" information (GitLab.com & 13.7+)
 - Regular issue and pull request comments
 - [Git Large File Storage (LFS) Objects](../../../topics/git/lfs/index.md)