Document strtonum()s behavior of setting errno to 0 when no error is found.

PR:             docs/143330
Submitted by:   Efstratios Karatzas (gpf dot kira at gmail dot com)
Discussed with: ru@
MFC after:      7 days

lib/libc/stdlib/strtonum.3

@@ -83,6 +83,8 @@ is set, and
 .Fa errstr
 will point to an error message.
 On success,
+.Va errno
+is set to 0 and
 .Fa *errstr
 will be set to
 .Dv NULL ;