sed.1: Adjust examples to fit in one line + SPDX

Examples were previously overflowing at MANWIDTH 80. Keep the same
complexity but use shorter phrases to make them easier to read. Tag
SPDX while here.

MFC after: 3 days
Reviewed by: bcr
Discussed with: emaste
Differential Revision: https://reviews.freebsd.org/D50867

(cherry picked from commit 3cb2a7e01e13c866d7c4bb55da8f229233c0cd7c)