explain the X-Okapi-Url header in the doc
explain the X-Okapi-Url header in the doc

(should have been in the previous commit)

funkymalc/okapi,julianladisch/okapi-acquisitions-poc,folio-org/okapi,julianladisch/okapi-acquisitions-poc,julianladisch/okapi-acquisitions-poc,folio-org/okapi,funkymalc/okapi