Document that readBuf.int16() is unsigned

It might make sense to rename it to uint16, but seeing that we're never
interested in signed 16-bit integers, it seems unnecessary.

buf.go

@@ -21,6 +21,7 @@ func (b *readBuf) oid() (n oid.Oid) {
 	return
 }
 
+// N.B: this is actually an unsigned 16-bit integer, unlike int32
 func (b *readBuf) int16() (n int) {
 	n = int(binary.BigEndian.Uint16(*b))
 	*b = (*b)[2:]