elf(5): document notes

elf(5): document notes

Document the Elf{32,64}_Nhdr structure, the sections/segments that
contain notes, and how to interpret them.  I've been lazy and only
included the GNU extensions here, especially as others are not
defined in the elf.h header file as shipped by glibc.

I've mostly used binutils, glibc, breakpad, and the GABI ELF spec
as sources of data for these fields.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>