mem(4): Improve ioctl section formatting

Add the missing .El which fixes the indentation of the memory range
definitions and operation. Add subsection headings to further clarify
this section. Do the same for the RETURN VALUES section, and mention
explicitly that MEM_EXTRACT_PADDR always returns zero.

Reviewed by: markj, 0mp, Pau Amma <pauamma@gundo.com>
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34574