as ODF 1.2 mentions such constaints of these functions, and
Excel's documentation also admits it with more specific remark:
<https://support.office.com/en-us/article/skew-function-bdf49d86-b1ef-4804-a046-28eaea69c9fa>
<https://support.office.com/en-us/article/skew-p-function-76530a5c-99b9-48a1-8392-26632d542fcb>

Change-Id: If19350997ddcb6f0fb36e29e090e260c3ae3d262
Reviewed-on: https://gerrit.libreoffice.org/61130
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>