style.mdoc.5: Point readers to style(9) regarding copyright headers

While here, fix the ordering of the elements in the copyright header per
style(9) and remove the text of the license as we already have the SPDX
tag here.

MFC after: 3 days

(cherry picked from commit 3cc7d67155dad57e0d10f02a584fda6c8f2ea0b2)