lib(4): Fix mdoc issues.

• Fix "mandoc -Tlint" warnings.
• Use the "Er" macro for errors.
• Add an extra newline for readability.
• Reference m_pullup(9) and m_defrag(9).
• Use the "Fx" macro when talking about FreeBSD versions.

Reviewed by: bcr
Approved by: re (gjb), krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17270