Make the PIOD_READ_AUXV operation more clear

Describe how to set piod_offs.

Sponsored by <The NetBSD Foundation>

lib/libc/sys/ptrace.2

@@ -1,7 +1,7 @@
-.\"	$NetBSD: ptrace.2,v 1.56 2017/01/15 22:18:11 kamil Exp $
+.\"	$NetBSD: ptrace.2,v 1.57 2017/01/25 17:12:56 kamil Exp $
 .\"
 .\" This file is in the public domain.
-.Dd January 14, 2016
+.Dd January 25, 2016
 .Dt PTRACE 2
 .Os
 .Sh NAME
@@ -285,9 +285,16 @@ Possible values are:
 See the description of
 .Dv PT_READ_I
 for the difference between I and D spaces.
+.Pp
 The
 .Dv PIOD_READ_AUXV
 operation can be used to read from the ELF auxiliary vector.
+The
+.Fa piod_offs
+argument sets offset withing the tracee's vector.
+To read from the beginning of it, this value must be set to 0 and casted to
+.Dv (void *) .
+.Pp
 A pointer to the I/O descriptor is passed in the
 .Fa addr
 argument to