Update excluded items for export/import instructions

What does this MR do and why?

The list of excluded items on Migrating projects using file exports is not exhaustive, but the title of the list seems to imply it is.

The following items are not exported:

This MR changes that line to read:

Items that are not exported include:

We may decide to further explain this by mentioning the iterative nature of GitLab, so I am open to suggestions!

cc: @bcarranza

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