Specific requirements should be included in the scope of the organization as required in IATF 16949 Clause 4.2.3. Therefore, the document that best suits is a Quality Manual or similar document in which an organization has defined the scope. The customer-specific requirements can change designed or actual scope of the organization, that is why it is important to use a document where an organization defines the scope.
Also, matrix indicating where within the organization Customer-Specific requirements are addressed should be included in the document. You can use a procedure or work instruction depending on your organization documentation scheme.