mdoc.7: Revert upstream changes to Lb/LIBRARY

Since this manual describes the mdoc syntax throughout the ecosystem,
I had to blend what we had before with what upstream is doing now.

Thanks: adrian
MFC after: 3 days (we shipped this doc in 15.0/14.4)
Reviewed by: ivy, mhorne, des, adrian
Discussed with: arch@
Differential Revision: https://reviews.freebsd.org/D56153

(cherry picked from commit 4e2bf6e90a65bee317e4dd379e098ba2eb5d8c0d)