add an example, similar to the one in strspn.3

src/lib/libc/string/strcspn.3

@@ -33,7 +33,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.\"	$OpenBSD: strcspn.3,v 1.2 1996/08/19 08:34:14 tholo Exp $
+.\"	$OpenBSD: strcspn.3,v 1.3 1999/06/04 16:39:27 aaron Exp $
 .\"
 .Dd June 29, 1991
 .Dt STRCSPN 3
@@ -65,6 +65,20 @@ The
 .Fn strcspn
 function
 returns the number of characters spanned.
+.Sh EXAMPLES
+The following call to
+.Fn strcspn
+will return 3, since the first three characters of string
+.Fa s
+do not occur in string
+.Fa charset :
+.Bd -literal -offset indent
+char *s = "foobar";
+char *charset = "bar";
+size_t span;
+
+span = strcspn(s, charset);
+.Ed
 .Sh SEE ALSO
 .Xr index 3 ,
 .Xr memchr 3 ,