em.4: add interfaces to name section

Improving first-glance comprehension that this manual page applies to
igb/lem interfaces provided by the em(4) driver.

While here, tag SPDX, link the framework, and shuffle a sentence to fix
linter warning.

Co-authored-by: mhorne
Reviewed by: mhorne
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1134

(cherry picked from commit 501f7e7bf847c66e3869b533805e76897e4685b9)