Fix doc strings of 'vc-version-diff' and 'vc-version-ediff'
* lisp/vc/vc.el (vc-version-diff, vc-version-ediff): Describe arguments in the doc strings. (Bug#35019)
* lisp/vc/vc.el (vc-version-diff, vc-version-ediff): Describe arguments in the doc strings. (Bug#35019)