top(1): Rework DESCRIPTION OF MEMORY section.

Due to markup issues, the DESCRIPTION OF MEMORY section is rather
unreadable; rework it a bit, using subsections for different lines of the
top output, and move it closer to description.

While here, pet manlint ordering other sections as expected.

Submitted by: Yuri Pankov <yuripv@yuripv.net>
Reviewed by: eadler
Approved by: re (gjb), krion (mentor)
Differential Revision: https://reviews.freebsd.org/D17369