Documentation about LFS objects not being copied when mirroring is active

Description

Projects can be mirrored from or to another repository. When activated, we mirror branches, tags, and commits. However, we do not mirror the LFS objects if the repository uses LFS, but nowhere in GitLab do we say that. More details are available about this here: https://gitlab.com/gitlab-org/gitlab-ee/issues/1252

Proposal

We need to add in the documentation about Mirroring a project that LFS objects are not copied.

Link to documentation: https://docs.gitlab.com/ee/workflow/repository_mirroring.html

Adding @seanpackham to the discussion.