Status/2023Q1/timerfd.adoc: Improvements

• Use man:...[] macro.
• Use ... delimiters for code.
• Remove line breaks before and after link.

Reported by: grahamperrin
Reviewed by: grahamperrin
Approved by: dbaio (co-mentor), carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D39265