[DOCS] Document impexp user TSconfig

Add the following information to impexp documentation:

- export functionality is restricted to admin users and
  users for which the new User TSconfig option
  options.impexp.enableExportForNonAdminUser has been
  set
- export functionality poses security risk as explained
  in patch https://review.typo3.org/c/Packages/TYPO3.CMS/+/74902
- the same restrictions apply to the import functionality
  (which is not new but has not been documented)

Resolves: #97899
Releases: main, 11.5, 10.4
Change-Id: Icb16dc97505a3bbe671357555bb786705121cb67
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75110


Tested-by: core-ci <typo3@b13.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>