Mention that stroul() et al *do* accept negative numbers, they

just (effectively) cast to unsigned.  ho@ OK

src/lib/libc/stdlib/strtoul.3

@@ -33,7 +33,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: strtoul.3,v 1.12 2003/03/06 18:19:26 kjell Exp $
+.\"	$OpenBSD: strtoul.3,v 1.13 2003/04/21 21:49:40 millert Exp $
 .\"
 .Dd June 25, 1992
 .Dt STRTOUL 3
@@ -83,6 +83,11 @@ The conversion is done according to the given
 .Fa base ,
 which must be a number between 2 and 36 inclusive
 or the special value 0.
+If the string in
+.Fa nptr
+represents a negative number, it will be converted to its unsigned equivalent.
+This behavior is consistent with what happens when a signed integer type is
+cast to its unsigned counterpart.
 .Pp
 The string may begin with an arbitrary amount of whitespace
 (as determined by