Tidy up the ports.7 manual page.

• Use "Dq Li" for inline commands as we do in other manuals.
• Pet "igor" and "mandoc -Tlint".
• Reword some parts for clarity.
• Add missing Xr macros.
• Reformat SEE ALSO to make the section more readable.

Reviewed by: eadler, krion, mat
Approved by: krion (mentor), mat (mentor)
Differential Revision: https://reviews.freebsd.org/D15350