style.mdoc.5: Improve description compatability

Use one line without child macros for document description, for optimal
compatability with manual tooling.

MFC after: 3 days
Reviewed by: carlavilla, imp
Approved by: carlavilla (mentor)
Pull Request: https://github.com/freebsd/freebsd-src/pull/1629

(cherry picked from commit bfe9be2253ef85674b37ae7cf3d2689f96678151)