ts(1): digest option is mandatory

not specifying the digest both on command line and in the config file
will lead to response generation aborting with

140617514493760:error:2F098088:time stamp routines:ts_CONF_lookup_fail: \
    cannot find config variable:crypto/ts/ts_conf.c:106:tsr_test::signer_digest
Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NMatthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8421)

(cherry picked from commit 29716a03e8c602d69d6c86558b2903b23edddd9b)

doc/man1/ts.pod

@@ -262,7 +262,7 @@ specified, the argument is given to the engine as a key identifier.
 =item B<-I<digest>>
 
 Signing digest to use. Overrides the B<signer_digest> config file
-option. (Optional)
+option. (Mandatory unless specified in the config file)
 
 =item B<-chain> certs_file.pem
 
@@ -460,7 +460,8 @@ command line option. (Optional)
 =item B<signer_digest>
 
 Signing digest to use. The same as the
-B<-I<digest>> command line option. (Optional)
+B<-I<digest>> command line option. (Mandatory unless specified on the command
+line)
 
 =item B<default_policy>