docs: update GitLab integration for external MR support

[tembo]

What changed

Updated the "MR comments" usage note in integrations/gitlab.mdx to reflect that @tembo now also responds on merge requests Tembo didn't create or previously sync — including MRs from outside contributors on an activated repository.

Why

The docs said the GitLab integration "Works on MRs created by Tembo and MRs created by your team," which no longer reflects the current, released behavior. Tembo can now lazily sync and create a session for an untracked ("external") merge request the first time someone mentions @tembo on it.

Evidence

• tembo/monorepo PR #10523, "Gitlab: support for tagging Tembo in external MRs" (commit 366ae86155), which adds apps/api/src/integrations/vendors/gitlab/externalMergeRequest.ts and wires it into the note-webhook handler (apps/api/src/integrations/vendors/gitlab/webhooks.ts) so a comment on a merge request without an existing PullRequest record creates one and starts a session.
• Covered by tests: apps/api/tests/integrations/vendors/gitlab/externalMergeRequest.test.ts and externalMergeRequestWebhook.test.ts.
• Confirmed still current: the in-app "What's new" changelog for the Aug 20, 2026 release (apps/web/components/app-sidebar/nav-footer.tsx, PR #10657) explicitly lists "tag Tembo in external GitLab merge requests" as a shipped feature.

Confidence

High — this is a merged, tested, and now-advertised (in-app changelog) capability with a direct code path (createExternalGitLabMergeRequestSession) that contradicts the docs' narrower claim. The fix is a single sentence correcting an outdated restriction; no positioning or wording judgment calls were needed.

cc @benja

Want tembo to make any changes? Add a comment with @tembo and i'll get back to work!

---

Note

Low Risk
Documentation-only wording change with no code or security impact.

Overview
Updates GitLab integration docs so @tembo is described as working on external merge requests (outside contributors) on activated repos, not only Tembo- or team-created MRs.

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[tembo]

Requesting review from @benja who has experience with the following files modified in this PR:

• integrations/gitlab.mdx