Clarify Settings migrated to projects generated from Templates

Problem to solve

Related to #461846 (closed)

When creating a project from a template, it's not clear which settings are copied.

Our documentation states the following is copied:

• Branches
• Commits
• Tags

But then also in the same section says:

To learn more about what is migrated, see Items that are exported.

Which adds many examples:

Project configuration

Merge requests

Merge request approvers

Push rules

etc...

In practice we can see some settings are copied over, but fail to work correctly as shown in #461846 (closed) for merge request rules when using a group of approvers.

Further details

Proposal

Clarify exactly what can be copied from a template, and what cannot