Document merge request limits on GitLab.com

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Close this issue

Capturing this conversation from Add documentation about diff versions limit (!184024 - merged)

None of the existing MR limits are documented under https://docs.gitlab.com/user/gitlab_com/, should we just add a new section there?

(Yes. Yes, we should. doc/administration/instance_limits.md for self-managed might also need attention, too?)