net_if: give more details about the bug being fixed

net_if: give more details about the bug being fixed

Jasper Lievisse Adriaanse reported that NetBSD 5.2 also has
the same bug.  Also, when writing configure tests that use
<net/if.h> (such as for 'struct ifreq'), it is useful to know
what pre-requisite headers are needed within that test.

* doc/posix-headers/net_if.texi: Add clarification.

Signed-off-by: Eric Blake <eblake@redhat.com>